eHam

eHam Forums => HomeBrew => Topic started by: AJ4SN on June 11, 2017, 09:04:47 AM



Title: noise from DDS VFO
Post by: AJ4SN on June 11, 2017, 09:04:47 AM
I just built a DDS VFO to replace the analog VFO in my homebrew radio. It's pretty noisy. With a dummy load connected to the receiver, there are numerous buzzes and tones present. The DDS VFO is built in a separate shielded box. I am using the VFO design of AD7C that incorporates an Arduino microcontroller and an AD9850 module. Anyone have suggestions for improving the noise performance of the VFO?

Thanks,

Stan
aj4sn


Title: RE: noise from DDS VFO
Post by: AC7CW on June 11, 2017, 03:26:46 PM
Perhaps adding a lpf between the vfo and the radio would help? Ground noise is a possibility, could play with the wiring between the vfo and the radio and see if the noise changes. Overloading or mismatching the input stage in the radio is something to investigate maybe....


Title: RE: noise from DDS VFO
Post by: KL7CW on June 11, 2017, 03:52:29 PM
It is not at all unusual to hear various DDS "noises" on a receiver connected to a dummy load.  Fortunately often these signals are below the band noise especially on the lower HF bands.  So if a birdie is say an S2 on 80 meters and the band noise is S3...usually not a problem.  If the reverse is true you need to do some trouble shooting.  So hook up an antenna and see if band noise obscures the birdie.  A BPF or LPF for the VFO frequency may help.  Double check your cables and shielding.  You could even try running your VFO off of a separate (battery) supply.  Perhaps others can give you some more ideas to try.
          Rick  KL7CW


Title: RE: noise from DDS VFO
Post by: WW7KE on June 11, 2017, 03:58:26 PM
Perhaps adding a lpf between the vfo and the radio would help?

Many switching mixers require square waves to drive them, so an LPF won't work in those cases.


Title: RE: noise from DDS VFO
Post by: VK3YE on June 11, 2017, 04:53:39 PM
It's important to work out how the noise is getting in.  

Is it getting through the RF output from the VFO, through badly decoupled power connections or direct pick-up?  

I discuss this and how I sorted my problem out at

https://www.youtube.com/watch?v=zOGzQDwFOag

Even if your home location has a lot of RF noise that overcomes anything internally generated it's still good to fix the problem.  

Then you can operate portable from remote locations and fully enjoy their RF quietness.

 https://www.youtube.com/watch?v=9n_QYo6yyd0



Title: RE: noise from DDS VFO
Post by: AJ4SN on June 11, 2017, 05:01:49 PM
Thanks so much for all the suggestions. I do have a LPF included with the design and, by coincidence, the VFO runs from a separate power supply. I will definitely look into the grounding and shielding situation. I may try to shield the LPF which is in the same enclosure as the rest of the DDS VFO. Peter, I did watch your videos. They were very informative. Thanks for taking the time to make them. I am curious about the impact of the VFO output level on the spurious emissions. I inserted a step attenuator between the VFO output and the radio, and dropping the drive level somewhat does help. Is there an appropriate drive level that minimizes spurs? I am using a minicircuits TAK-3H mixer, and I try to keep the drive level as close to the 17dBm as possible. I understood that was the best way to have a strong mixer. Am I off base here?


Title: RE: noise from DDS VFO
Post by: VK3YE on June 11, 2017, 05:09:06 PM
I'm not that familiar with that mixer but excessive drive could cause problems.  

A lot of literature on diode mixers emphasise the importance of proper termination and good impedance matching over a wide range of frequencies. Discussed extensively in Experimental Methods for RF Design etc.

Also while it's no substitute for removing the noise in the first place an audio amplifier chain that has a limited audio bandpass (ie 300 - 3000 Hz) with a roll off below and above can reduce the effect of hum etc.


Title: RE: noise from DDS VFO
Post by: VK4FFAB on June 11, 2017, 11:48:10 PM
Thanks so much for all the suggestions. I do have a LPF included with the design and, by coincidence, the VFO runs from a separate power supply. I will definitely look into the grounding and shielding situation. I may try to shield the LPF which is in the same enclosure as the rest of the DDS VFO. Peter, I did watch your videos. They were very informative. Thanks for taking the time to make them. I am curious about the impact of the VFO output level on the spurious emissions. I inserted a step attenuator between the VFO output and the radio, and dropping the drive level somewhat does help. Is there an appropriate drive level that minimizes spurs? I am using a minicircuits TAK-3H mixer, and I try to keep the drive level as close to the 17dBm as possible. I understood that was the best way to have a strong mixer. Am I off base here?

The AD9850 should not be putting out 17dbm, it should be putting out -6dbm and then have a amp and buffer stage following it to bring the signal up beyond what your mixer requires. 17dbm is 5v p-p the sine wave output of the AD9850 is not much more than 300mV p-p.


Title: RE: noise from DDS VFO
Post by: N3QE on June 12, 2017, 06:30:08 AM
Constructing a good DDS VFO for receive use is in fact very difficult. Spurs from the DDS must be filtered out by tracking a tracking PLL and bandpass filters. NONE of this is present in the AD7C design and the majority of the buzzes and whines you are hearing are the result of the DDS spurs you would fully expect to hear by using a raw unfiltered DDS as the LO.

I think the AD7C design would be usable for a QRP transmitter where some tuned circuits in the amplifier chain helped clean up the worst of the spurs, at least the ones that were completely out-of-band. You would still be left with a number of in-band spurs that could only be cleaned up by a tight tracking PLL and even more filtering. Above the QRP levels these would be absolutely necessary to not pollute the whole band.


Title: RE: noise from DDS VFO
Post by: K9MB on June 12, 2017, 07:42:56 AM
N3QE, very good points.
I am just getting parts together to build one of these things, but was thinking last night that the thing has to be very noisy and that I need to build up a nice stable and "quiet" VCO and phase lock it to the DDS, with appropriate filtering to eliminate jitter and spurs. Might make sense to put the pristine VCO in a separate compartment from DDS.

Close in noise may still be present, though reduced by loop filter, but spurs away from the channel can be pared off, I think.

I am surprised that this issue has not been addressed. I am new to this kind of PLL.

I used divide by N traditional loops with down mixers and always used a solid crystal oscillator for first conversion in my vhf and uhf weak signal receiver designs.

At HF  where noise figure in the front end is of no importance, the bigger issue is spurious out of bandpass signals that cause noise and heterodynes as one tunes across a band already full of outside intereferences.
A tracking vco should serve nicely there.


Title: RE: noise from DDS VFO
Post by: N3QE on June 12, 2017, 09:07:58 AM
I am surprised that this issue has not been addressed.

Again, at the hobbyist QRP transmitter level, I think the AD9850 with just the usual transmit chain bandpass filtering may be acceptable. But only at QRP levels. Above QRP levels, the in-band spurs will quickly earn a bad reputation among neighboring hams. I think most homebrew users never get past the QRP stage.

DDS's are very often important parts of modern ham synthesizers. And synthesizer noise (not just DDS but also simpler PLL) took DECADES for the major ham manufacturers to address adequately for good receive performance. So this has been largely addressed but the devil is in the details. Even today, the best synthesizers still produce noticeable spurs and phase noise that can be measured as adversely affecting receive performance.

Real-world ham rigs that use a DDS as part of the synthesizer, use a DDS running at a very high clock (sometimes bigger than 1GHz) for fine frequency control feeding a mixer that is part of a PLL loop that also uses more conventional divide-by-n synthesizers for the coarse steps. The DDS is run over a rather limited output frequency range and has superb tight bandpass filtering after it to make sure all the spurs are down far enough. This is in addition to the cleanup provided by the PLL loop and divide-by-n factor. Often there are multiple PLL loops in real ham synthesizers, not just one, to provide even further cleanup while still offering frequency agility.

Good synthesizer designs to research, would be the Elecraft K3 synthesizer and the Ten-Tec Eagle synthesizer. Even then, for example, Elecraft recently upgraded its synthesizer to take down even further close-in phase noise that was mearuably affecting rig performance. So there is always room for improvement even with the very best the industry has developed.


Title: RE: noise from DDS VFO
Post by: AC7CW on June 12, 2017, 01:10:56 PM
Perhaps adding a lpf between the vfo and the radio would help?

Many switching mixers require square waves to drive them, so an LPF won't work in those cases.

The VFO generates a sine wave by calculating the value. It's stepped iow, and always requires some filtering to have a cleaner waveform. We really don't have much to go on in this question so i tossed out some general idea of what I'd look for first...


Title: RE: noise from DDS VFO
Post by: AJ4SN on June 12, 2017, 04:54:21 PM
Here's a little more information on the DDS VFO. I did include a feedback amplifier and a low pass filter taken right from Experimental Methods in RF Design (page 6.39 revised first edition). That enabled me to raise the output level to 15.9 dBm. I use this 5-5.5 MHz signal to drive my main station receiver which has a 9 MHz IF. It's a homebrew unit that uses a minicircuits TAK-3H mixer. A dual gate MOSFET serves as the RF amplifier in the receiver. The mixer is followed by W7ZOI's "hycas" IF and a 2.4 kHz 8-pole crystal filter as well as an 8-pole 500 Hz filter for CW. Another TAK-3H serves as the product detector. The receiver has been in use for several years, and it performs well with its analog VFO; however, the tuning is just a little too fast for CW, and I thought that the DDS VFO would be a nice improvement. It sounds like the DDS project may end up as another signal generator for the shack (I can always use another one). I just didn't expect it to produce so much noise! Apparently, a DDS system that is comparable to the analog VFO in noise performance is relatively complex, and I might be fighting above my weight class trying to build one! I really appreciate your feedback and suggestions, though. Thanks for taking the time to help. 

73,

Stan


Title: RE: noise from DDS VFO
Post by: G4AON on June 13, 2017, 07:56:58 AM
A couple of points. The drive level to a mixer should be measured into a 50 Ohm termination and not measured with an oscilloscope across the mixer osc input pin, which can give misleading results.

I use an SDR Kits Si570 based synth in a few projects. They are relatively quiet and low cost. If you get one, ask for firmware version 4.21 as the current listed one (4.20) removes the IF offset when you engage dial lock! 4.21 works OK. The output of an Si570 is a square wave of about +10dBm, with a 3dB attenuator it drives a level 7 mixer just fine.

http://sdr-kits.net

73 Dave


Title: RE: noise from DDS VFO
Post by: YL3GND on June 13, 2017, 02:38:32 PM
"mixer is followed by "hycas" IF and crystal filter"...really? Sounds pretty bad composition. Mixer with an IIP3 around +23dBm and OIP3 around +29dBm is followed by something so crappy as "hycas" amplifier.. Or there is a typo and crystal filter is in between?

Egmont


Title: RE: noise from DDS VFO
Post by: AJ4SN on June 13, 2017, 06:15:42 PM
Egmont,

You are correct, the crystal filter precedes the IF amp. The IF amp is the hybrid-cascode designed by W7ZOI. I was under the impression that it is a decent IF strip. Are there some deficiencies with the design? I'm always interested in improving my receivers.

Stan


Title: RE: noise from DDS VFO
Post by: N3QE on June 13, 2017, 07:31:45 PM
You are correct, the crystal filter precedes the IF amp. The IF amp is the hybrid-cascode designed by W7ZOI. I was under the impression that it is a decent IF strip. Are there some deficiencies with the design? I'm always interested in improving my receivers.

The hycas is a good clean design.

You are right, the weak link in your setup is the DDS VFO.

I am curious, do small changes like a single step frequency change in the DDS VFO cause a complete change in the buzzes and whines?

If so, you are getting the spurs.

You could help clean it up a lot, by getting, say, a 20MHz output from the DDS VFO, filtering it up there with a 20MHz bandpass, and dividing by 4 followed by a simple low pass to get to 5MHz. This may clean up many (but not all) of the worst spurs you hear. I think a PLL tracking loop could have similar benefits too.


Title: RE: noise from DDS VFO
Post by: KB1GMX on June 14, 2017, 09:57:03 AM
Likely the noise being detected is not the DDS fault.  DDS even the lowly 9850 has how jitter (unless its reference is junk).
It will have spurs but those tend to be better than 60db down especially int eh under 20mhz realm.  For a 5-5.5mhz VFO it can be very good.

So based on that (and experience using the 9850) if you getting noises and beeps its entirely likely you have oscillation in the
amplifies(s) used to get to 16dbm (Tak3H is a high level DBM using a nominal 17dbm drive).  You may also have  the post mixer  amp doing unstable things as well as the HyCas (the published design is over 100db of gain, do you really need that much?).  Its very easy to combine stable components to get an unstable result if care, shielding, proper grounding, and bypassing are not taken into account.

One topic not handled separately but is part of the text in EMRFD is that shielding is part of any design of a high performing receiver.  That applies to many of the RF circuits to keep them from picking up signals that produce unwanted results.  Keep in mind that a 16dbm signal for the mixer is a 50mW signal and even short leads will easily radiate enough that a stage a few inches or less away will hear it.  If your serious  every stage should be in a box with 50 ohms in and out and those boxers in the bigger box (case) and signals between them that are not DC shielded and bypassed as needed.

Using a high level mixer requires a fair degree of care in the overall design as the leakage products are significant.  Also its alone does not guarantee a good receiver (resistance to overload/intermod) as everything downstream but prior to the selectivity (crystal filter) must be able to handle big signals or bad things result as mentioned.

In this case lacking a lot of info but having used the 9850 DDS blaming it for the "noises" is likely blaming the cow for soggy cereal.  Its likely not the cause but may be a symptom of many combined design or assembly faults.

FYI the DDS has a typical output of 200Mv into a 200 or so ohm load so most DBM require amplification and
should be followed by a band pass or low pass filter as appropriate.

The HyCas amp is a good design but care must be used as wide open gain is 100-120DB, that much gain must be respected as oscillation can be the result.  I've used it and it works well but its input does not properly load a filter without some form of matching network. (L or tuned transformer).

High level mixers (tai-3H or SRA1h) need to be followed with a very robust post amp with the ability to handle large signals as LO leakage alone will be around -30dbm (assume a 45db Lo suppression in the DBM) and can easily be larger.  Overload in this stage will be fatal to good performance. 

Allison/kbqgmx



Title: RE: noise from DDS VFO
Post by: AJ4SN on June 14, 2017, 08:24:40 PM
Allison,

Thanks for your reply. I hadn't thought about how much power 17 dBm represents. 50 mW almost rises to the level of a QRP transmitter! I see what you mean about taking care with the design. I didn't set out to build a receiver with high dynamic range (I'm not nearly technical enough to do that). I just found the TAK-3H mixers on the cheap, and I read an article about the hycas IF. Just a point of clarification, the receiver has worked well with the analog VFO for a several years. I've tried to include a picture of the inside of the receiver with the IF shield off. It should also be on my profile page. You motivated me to update my profile with the latest gear! I'm going to look at the amp that I built for the DDS to see if it might be oscillating, and I'm also going to use the receiver with the LO drive level reduced. I think it should work ok, although there may be more loss in the mixer. I probably have more gain than I need anyway. By the way, I love the expression, ". . . blame the cows for soggy cereal." I think I'm going to use it at work!

73,

Stan

(https://s3.amazonaws.com/files.qrz.com/n/aj4sn/DSC_0661.JPG)


Title: RE: noise from DDS VFO
Post by: G4AON on June 14, 2017, 11:42:07 PM
Stan, if that open board includes your Hycas IF, it could do with being completely shielded. A Hycas board is sensitive to sub microvolt signals and maybe why you hear some of the DDS noise.

By the way neat assembly!

73 Dave


Title: RE: noise from DDS VFO
Post by: G3RZP on June 15, 2017, 02:11:27 AM
Quote
Also its alone does not guarantee a good receiver (resistance to overload/intermod) as everything downstream but prior to the selectivity (crystal filter) must be able to handle big signals or bad things result as mentioned.

You need to keep gain down prior to any crystal or mechanical filter, because they produce intermodulation products, and it is relatively easy to have a receiver front end where the IMD performance is dominated by the filter. On occasion, IMD performance can be improved by exchanging the input and output connections, as the filter may well not be symmetrical as far as IMD is concerned. Also, high frequency (e.g. 9MHz) filters are generally worse on IMD than lower frequency e.g. 1.4 MHz filters. Some 455kHz mechanical filter had intercept points as low as -12dBm.


Title: RE: noise from DDS VFO
Post by: YL3GND on June 15, 2017, 04:11:57 AM
Stan, if there is a problem not only associated with spurs, but also noise (lowered sensitivity), than You have equation with 3 multipliers:
1.   DDS noise itself
2.   LO leakage to IF (including noise, depends on mixer construction)
3.   Given level of LO
Apparently the 4th one also counts: IF sensitivity (from the output of mixer), but let’s assume it’s a constant.

This is why I was asking about the circuitry (it’s pity that on eham you can’t put pics directly..) – may be there is no point in putting in +17dBm mixer. Instead if You would’ve been using +7dBm one with a similar LO isolation, You would get the noise 10 dB down.

I mean, if there is some circuitry in the front end that ruins dynamic range more than a mixer, there is no point in particular mixer.

Could You, please, tell (better draw also!) more about your front end circuitry? What is the sensitivity without a preamp? Does preamp exists only for to use “rarely only on 10m band” or..?

Egmont


Title: RE: noise from DDS VFO
Post by: YL3GND on June 15, 2017, 04:45:38 AM
Just acknowledged there are some more valuable thoughts in there – esspecially from Allison KB1GMX, about blaming the cow for soggy cereal. That’s on spot!

About cascode amps (like hycas): I am using one (with 2bjt’s) myself, but only where signals never grown big – in the 80m band ARDF receiver with 6” diameter loop antenna. Where there are stronger signals (even inband ones, after crystal filter) it is counter intuitive to use it: the stronger the signal, the less linear is amplifier due to lowered current by applied AGC.

As I got, Stan, You do not use post mixer amp?

Egmont


Title: RE: noise from DDS VFO
Post by: N3QE on June 16, 2017, 03:19:20 AM
About cascode amps (like hycas): I am using one (with 2bjt’s) myself, but only where signals never grown big – in the 80m band ARDF receiver with 6” diameter loop antenna. Where there are stronger signals (even inband ones, after crystal filter) it is counter intuitive to use it: the stronger the signal, the less linear is amplifier due to lowered current by applied AGC.

Well then just use 1 or 2 (not 3) stages in the hycas.

A lot of us just habitually put a 20dB or 40dB attenuator on whenever we switch to 80M anyway. I work more countries every year on 80M, than just about anyone else in North America, and I work them all with the attenuator on :-).


Title: RE: noise from DDS VFO
Post by: KG7TR on October 05, 2017, 09:05:43 PM
Everything N3QE stated in his 12 June 17 responses is absolutely true.  I have been experimenting for over a year with a vacuum tube homebrew receiver that started out as a clone of a Collins 75S-3. The original plan was to use Arduino controlled AD9850 DDS modules to replace the first crystal heterodyne oscillator, PTO (VFO), and BFO.  The rest of the circuitry is all vacuum tubes. I used LMH7603 current feedback op amps to get the oscillator signals up to a couple of volts RF to inject into the cathodes of the receiver's mixers, just like the 75S-3.

When I finally got all the circuits built and working results were most disappointing, with birdies and digital sounding spurs all across the spectrum. Some were up to an S3 or S4 level of annoyance. The scope patterns from the AD9850/LMH7603 chips looked like beautiful sine waves, but a little more internet research confirmed my suspicions that there was some low level junk getting into the IF. The first LO/first mixer and PTO/second mixer are followed by two IF amps with a combined gain of at least 80db. The spurs are perhaps only 50-60 db down from the oscillator carriers, so they come blasting right through.  Next I tried si5351 PLL modules, and results were not any better.

These chips are remarkable for what they can do, but they were never designed for sensitive HF receiver applications.  I finally settled on a crystal controlled first oscillator using cheap microprocessor crystals, and an analog tube type VFO.  The BFO does use an Arduino controlled si5351 module followed by an LMH7603 amplifier.  At that point the spurs are too low to make any difference. The radio is working pretty well now. The only birdies are due to harmonics of the VFO, and they are equivalent to less than .1 microvolts at the antenna.

I really wish I had done more research ahead of time, but I did learn a lot in the process.  I echo other's sentiments that these chips might be OK for QRP radios, but not for sensitive receiver applications or high power transmitters.


Title: RE: noise from DDS VFO
Post by: G4AON on October 06, 2017, 02:34:53 AM
Next I tried si5351 PLL modules, and results were not any better.
<SNIP>
These chips are remarkable for what they can do, but they were never designed for sensitive HF receiver applications.
<SNIP>
I use an Si570 synth in my home brew RX and it's fine. They are also used in the Elecraft KX3 and K3S. The KX2 uses an Si5351. So I doubt they are not suitable for sensitive HF receivers.

I suspect the issue is one of shielding and/or effective decoupling. My home brew RX could do with better shielding but it's adequate. Having a decoupling capacitor only half an inch from a ground point makes quite a big difference when dealing with signals in the +10dBm region, as does assuming a decoupling capacitor must be OK without checking the RF level across it (a 100nF ceramic cap might not be as effective as you think).

On the other point about the Hycas board design, mine works absolutely fine although I've done a couple of tweaks... R6 is shown on the circuit as being 270 Ohm, with that value high ambient temperatures cause the AGC Voltage to drift. Using 220 Ohms stops that drift. There are some adjustments to the overall gain that can be done easily with the damping resistors across the RF chokes, W7ZOI mentions this on his site and details are also on my site: http://www.qsl.net/g4aon/g4aon_rx/

73 Dave


Title: RE: noise from DDS VFO
Post by: YL3GND on October 07, 2017, 05:41:44 AM
Very interesting, Dave! About KX2 using Si5351, have not heard this before. Also I am interested in K3S LO circuitry (contemporary Elecraft is hiding their circuitry) - does it have PLL loop with Si-570, or it goes straight out of the chip?

Egmont


Title: RE: noise from DDS VFO
Post by: YL3GND on October 07, 2017, 06:58:18 AM
About IF amps: As I stated earlier, its basically a bad technical idea (only positive - easy to implement) to steer amplification down by reducing current in amplifier's active device. That's why better designs are using passive attenuators between amplification stages.
I am using one MC1350 on one of my portable rigs myself  :P , I would never state it's a great IF amplifier, despite it does what I need to be done in this rig.

N3QE: absolutely wrong - its better to use more such a bad IF amps in a string, as than You don't need to shut them completely down for larger signals.

Recently I came across pirate copy of "EMRFD" book of W7ZOI and others. What he wrote in chapter 6.2 about IF amps is erroneous at times. Like statement in the page 6.17 : "However intercept degrades slower than gain, so IMD products are allways decrising with gain reduction". The numbers given by himself tells us the opposite.

A lot of fairy tails not backed by measurements is circling around.

Egmont 


Title: RE: noise from DDS VFO
Post by: G4AON on October 07, 2017, 07:13:17 AM
Egmont, the K3S circuits are published on the Elecraft site.
http://www.elecraft.com/

Regarding EMRFD, several circuits aren’t thought through. There is a 2 stage basic amp that even I was able to redesign to have a flat frequency response, plus a few others. Spice modelling works wonders!

73 Dave


Title: RE: noise from DDS VFO
Post by: G3RZP on October 07, 2017, 03:52:12 PM
Over 40 years ago, I did a design for a 1.4 MHz  IF amplifier using discrete transistors (2N5179 I think) , in which AGC was applied using a PNP transistor in series with the emitter bypass capacitor. By taking the base positive, the PNP transistor was turned off, the negative feedback increased and the IMD performance improved as the gain was reduced. For the life of me, I can't remember why we didn't end up using it in production.

The usual way in ICs was to use a long tailed pair and divert the current from one transistor to the other. An improvement over just reducing the current in a single transistor, but not ideal.


Title: RE: noise from DDS VFO
Post by: YL3GND on October 08, 2017, 02:45:07 AM
Yes, G3RZP, exactly this is the way how MC1350 works.

My guess about this transistor circuit: May be input and/or output impedance changed too much with gain adjustment?

Thanks, Dave! They had published these schematics only 2 month ago. I was unaware of this. Do You know something more about Si5351 usage in KX2 ? It would be interesting to know a bit more.

Egmont


Title: RE: noise from DDS VFO
Post by: G4AON on October 09, 2017, 12:22:28 AM
Thanks, Dave! They had published these schematics only 2 month ago. I was unaware of this. Do You know something more about Si5351 usage in KX2 ? It would be interesting to know a bit more.

Egmont
Sorry Egmont, all I know is what is shown on the KX2 diagrams on the Elecraft site. I have a KX3 and a K3 with the updated synths, but don’t have a KX2.

73 Dave


Title: RE: noise from DDS VFO
Post by: ZENKI on October 09, 2017, 01:40:07 AM
Dont look at the early K3 design. The design has terrible SPI bus noise. Elecraft did produce a mod for this noise that works OK with the old synthesizer. Upgrading to the new synthesizer is no guarantee that you can get rid of the bad layout in the K3's SPI bus lines.

 I have 3 old K3's  and upgrading all the  synthesizers to the new synthesizer including  the sub receiver made matters worst.  I have tried 8 of the new synthesizers already! I know of at least 10 people at the club with old K3's who have this SPI bus noise problem with  the new synthesizer. My advice is not to buy a old K3 and expect to upgrade to the new synthesizer you might get unlucky and end up with horrible noise as you spin the VFO.

I will dump these old K3's once something more viable with knobs comes along, maybe the K4.  DDS noise is evident on most designs if you know how to look for it.


Very interesting, Dave! About KX2 using Si5351, have not heard this before. Also I am interested in K3S LO circuitry (contemporary Elecraft is hiding their circuitry) - does it have PLL loop with Si-570, or it goes straight out of the chip?

Egmont


Title: RE: noise from DDS VFO
Post by: G3RZP on October 10, 2017, 07:35:13 AM
The MC1350 is very old now. The device was meant for TV receivers: the same chip in the professional version in a metal can was the MC1590. Plessey produced the SL6140 which was basically the same thing but with much greater bandwidth.

As far as my discrete transistor IF was concerned, variation of input impedance of each stage didn't matter except for the input stage, which was low a noise fixed gain buffer so its impedances were defined.