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Author Topic: SB-301 LMO to actual tuned frequency display?  (Read 3544 times)
KD4SBY
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Posts: 223




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« on: January 01, 2013, 09:06:22 AM »

I like to display the actual frequency my SB-301 is tuned to, but have some trouble figuring out how to do that. I am using a frequency counter that has the capability of entering an amount in memory that will either add or subtract this amount from the input frequency and display the difference. I thought that I could do this with this receiver, but when I calculate the frequencies involved, I come out with different figures all the time. For example, when the radio is tuned to 7.087MHz, the LMO shows 5.416MHz, difference = 1.671MHz. when it is tuned to 7.268MHz, LMO=5.235, difference= 2.033MHz. The 1st mix Xtal = 15.895, IF = 3.395, passband filter = 8.395-8.895MHz. (The frequencies tuned to were verified by my main radio with a digital readout)
Can anybody tell me if it is possible to set my counter to a frequency to add or subtract to get the actual frequency the radio is tuned to? I must be doing something wrong or I am overlooking something. Or it is just not possible to do it with my counter.
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AD4U
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« Reply #1 on: January 01, 2013, 09:47:25 AM »

Heathkit offered a SB 650 external digital display.  As I remember it used three different frequencies generated inside the SB 301, delivered to the unit by three separate cables, processed them inside the SB 650, and then gave a digital readout if the frequency.  Using just a counter to get digital frequency readout, is not simple.

I have a "minty" SB 301 that I restored.  It has all 3 filters.  Ironically I am listening to 40 meters on it as I write this.  If constructed properly, it is a great receiver, even by today's standards.

Dick   AD4U
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AC5UP
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« Reply #2 on: January 01, 2013, 10:15:57 AM »

If you haven't downloaded the partial SB-301 manual available from Vintage Radio, you can find it here

On the first page you'll see a chart of mix frequencies used by the radio... In the case of 40 Meters, the 1st mixer is rockbound at 15.895 MC. A desired station on 7.200 MC would then mix to 8.695 MC and be passed through the 1st IF to the 2nd mixer. The LMO at 5.300 MC would bring that down to the 2nd IF frequency of 3.395 MC. I worked the math, the numbers are good, if you can offset the LMO relative to 3.395 MC's you might find a path to where you want to go...

Where things get dicey for what you're trying to do is in compensating for two offsets with one magic number. In the case of 40 Meters that might be possible (I'm on holiday today, don't ask me to think...) but what happens above 15.895 MC's where the injection goes low side instead of high?

In any case, what you're looking for is two offsets in cascade and as AD4U mentioned that's not easily done with a single counter.
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G3RZP
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« Reply #3 on: January 01, 2013, 10:46:46 AM »

The problem is that the VFO tunes backwards. The Heath counter, if I remember right, counted UP the crystal oscillator, counted DOWN the VFO and counted DOWN the carrier osc. But most counters count UP. You could mix the VFO with say 6 MHz and then count 500 kHz to 1 MHz with a "subtract 500kHz" on your counter, but the errors could be quite large because of errors in the crystal oscillators.
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N2EY
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« Reply #4 on: January 02, 2013, 06:06:08 AM »

G3RZP is correct. You need a presettable down-counter, because not only is the LMO offset, it tunes backwards.

73 de Jim, N2EY
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W9GB
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« Reply #5 on: January 02, 2013, 07:59:52 AM »

AADE makes an LCD frequency display unit (witout enclosure) for the Heathkit SB-series.
Product uses the 3 RCA (phono) jacks on rear panel and has a PIC processor for operation.
Model# DFD2-301
http://aade.com/dfd.htm
« Last Edit: January 02, 2013, 08:03:05 AM by W9GB » Logged
KD4SBY
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Posts: 223




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« Reply #6 on: January 02, 2013, 11:50:28 AM »

G3RZP is correct. You need a presettable down-counter, because not only is the LMO offset, it tunes backwards.

73 de Jim, N2EY
The backwards tuning is not the problem. My counter has several memory locations where I can store a positive or negative number, were then the counter will either add or subtract this amount from the input frequency. The problem is that I can not derive at a figure that I can add or subtract from the LMO frequency to display the tuned frequency.
(Normally you accomplish this by taking the IF frequency and adding or subtracting that from the VFO Frequency, but that does not seem to work with the scheme the SB-301 uses.)
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K4BNC
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« Reply #7 on: January 02, 2013, 12:17:17 PM »

Per your first post, the sum of the 40M receive frequency and the VFO frequency is a constant. (The constant is approximaely the difference between the two oscillator frequencies which is consistent with the explanation of needing to combine all 3 to get the receive frequency.)  Given that this is a known constant that is fixed by the two crystal oscillators. you would nned to subtract the VFO from that constant to get your readout. 
Thus having the ability to add or subtract a constant in your counter from the signal being counted only works if you can count down - the direction of tuning does matter.
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WB6BYU
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« Reply #8 on: January 02, 2013, 04:52:04 PM »

Quote from: KD4SBY

The problem is that I can not derive at a figure that I can add or subtract from the LMO frequency to display the tuned frequency.



What if you subtract the LMO frequency from a constant number, instead
of the other way around?
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KD4SBY
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Posts: 223




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« Reply #9 on: January 03, 2013, 06:28:10 AM »

Thanks for the replies, fellows. There is enough information and suggestions here to see if I can accomplish what I want.
Ironically, my LMO dial readout is more accurate than I thought. To my surprise, I can actually read the frequency directly from the circular dial and be very close. I checked that by tuning on a station, reading the dial, and then using that reading to find the station on my Icom. Within a small error, which can be interpreted as an reading error, I was right on the station, every time. I never checked that until now, and did not expect it being so accurate.
It is just that a digital readout is so much more convenient.
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N2EY
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« Reply #10 on: January 03, 2013, 02:50:34 PM »

Here's how the Heath heterodyne scheme works, using 40 meters as an example:

1) The incoming signal is subtracted from the heterodyne oscillator signal. On 40, the heterodyne oscillator is on 15,895 kc, so a 7000 kc. signal comes out of the first mixer at 8895 kc. and a 7500 kc. signal comes out at 8395 kc. The first IF range is 8895 to 8395 kc.

2) The LMO signal is subtracted from the first-IF signal to give 3395 kc, the second IF. When the LMO is at the top end of its range (5500 kc), the 8895 kc signal is converted to 3395 kc., and when the LMO is at the low end, the 8395 kc. signal is converted to 3395 kc.

So for a digital dial to work, the formula is this:

F (het osc) - F (2nd IF) - F (LMO)

Since the first two are fixed, one can get the frequency by presetting the counter to the heterodyne oscillator minus the IF, then downcounting (subtracting) the LMO frequency. On 40, this works out to 15895-3395 - LMO, or 12,500 minus the LMO frequency.

No other combination will work.

73 de Jim, N2EY
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G3RZP
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« Reply #11 on: January 04, 2013, 12:32:54 AM »

But you actually need to count the crystal heterodyne osc and the crystal carrier insertion oscillators to remove errors in them, especially as the CIO frequency depends on the sideband in use. Otherwise, you can be more than 2 kHz out.
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N2EY
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« Reply #12 on: January 04, 2013, 05:46:31 AM »

But you actually need to count the crystal heterodyne osc and the crystal carrier insertion oscillators to remove errors in them, especially as the CIO frequency depends on the sideband in use. Otherwise, you can be more than 2 kHz out.

That's true, but it's easily accomplished by tuning to a single known frequency for each band/mode and adjusting the preset until the digital display reads correctly. Similar to calibrating an analog dial - but you only need to do it once per band/mode.

I know all this because, back in 1974-75, I designed and built a digital-dial adapter that could work with any superhet HF receiver. It used 74192 presettable up-down counters and a diode matrix preset system that allowed up to five presets. Could be expanded to as many as desired by adding diodes and positions on the selector switch.

73 de Jim, N2EY
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KD4SBY
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Posts: 223




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« Reply #13 on: January 04, 2013, 06:34:01 AM »

But you actually need to count the crystal heterodyne osc and the crystal carrier insertion oscillators to remove errors in them, especially as the CIO frequency depends on the sideband in use. Otherwise, you can be more than 2 kHz out.

That's true, but it's easily accomplished by tuning to a single known frequency for each band/mode and adjusting the preset until the digital display reads correctly. Similar to calibrating an analog dial - but you only need to do it once per band/mode.

I know all this because, back in 1974-75, I designed and built a digital-dial adapter that could work with any superhet HF receiver. It used 74192 presettable up-down counters and a diode matrix preset system that allowed up to five presets. Could be expanded to as many as desired by adding diodes and positions on the selector switch.

73 de Jim, N2EY
I came to the same amount of 12500 when trying to figure out what to subtract the LMO frequency from, but I still had problems with inaccuracies when checking the actual frequency with my Icom. The explanation of G3RZP explains why. I indeed had errors in excess of 2kHz which I could not account for, and that was the reason of my original posting. I just could not figure out why I was getting those erroneously readings!

Thanks fellows for solving that problem for me! It is now just a matter of correcting the error by empirical means for each band and then storing the corrected amounts in the counter memory.
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G3RZP
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« Reply #14 on: January 04, 2013, 07:08:48 AM »

I think you will need different numbers for each band for USB, LSB, CW RX and CW TX.
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