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Author Topic: 100Mhz Doubler  (Read 2582 times)
GM8UPI
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Posts: 84




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« on: October 30, 2010, 01:57:50 AM »

Looking for a simple circuit to act as freq doubler, with I/P 70Mhz to 75Mhz.
I own an HP Sig Gen/Sweeper which covers 1Mhz to 110Mhz with max output of +10dBm, and an HP Spec Analyser. I need to do an alignment job on 2 Mtr rig that requires a sweep input injected at the rx antenna, sweeping across 144 to 148 Mhz, with a level of a few mV into 50 Ohms.
Anyone help me with a basic/simple circuit to accomplish this ? Dead bug construction in die-cast box, would be ideal for me. (I have experience of construction up to 500Mhz and above.)
Doubler output purity need not be great.

Dave
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G3RZP
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« Reply #1 on: October 30, 2010, 02:33:11 AM »

Use a double balanced mixer - 4 diode type. Feed the tracking generator to two of the ports, get double the frequency from the third port. Something like an MD108 works, or make your own, although in that case, you need to be a bit careful with the transformers.
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GM8UPI
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Posts: 84




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« Reply #2 on: October 30, 2010, 03:21:37 AM »

Thanks. MD108 sounds good. Any idea of UK suppliers?
Have done a search on i/net but no luck so far.

Dave
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AD5X
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« Reply #3 on: October 30, 2010, 04:20:34 AM »

+10dBm is about 2Vp-p.  Why not try just putting a diode across the output of your signal generator and see how much 2nd harmonic you get with a little clipping?  I woulkdn't be surprised if you get enough to do the job.  Use a 1N5711 schottky diode.

Phil - AD5X
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GM8UPI
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Posts: 84




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« Reply #4 on: October 30, 2010, 04:59:31 AM »

Hi,
The +10dBm is the max o/p of the HP sig gen for info only.
I suspect just using a diode would produce the 2nd harmonic at low levels which may be usable as input to RX at 146 Mhz, but I would have thought the fundemental at 73 Mhz would possibly damage the front end, without building a very good filter. (I suspect the RX front end may be fairly "open".)
May be wrong on this. Perhaps I shall give it a go.

Thanks.

Dave

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WB6BYU
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Posts: 13243




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« Reply #5 on: October 30, 2010, 07:12:45 AM »

If you are building your own you can simply use a full-wave rectifier circuit - just as in a mains power
supply (except, of course, for the input transformer, and you want to capacitively couple the output
rather than adding filter capacitors.)  I believe this is essentially what you get when you apply the
same frequency to both inputs of a DBM.

A square wave has high levels of odd harmonics, so running the signal generator into a logic gate
at, say, 16 MHz (perhaps something in the 74HC series?) and adding a simple tuned circuit to select
the desired harmonic should give you enough drive for calibrating the receiver.

The approach that we've used to extend the range of our signal generators is to use a canned
oscillator into a DBM to make an up-converter.  Stock oscillators are often available at 50 or 100 MHz,
which makes the frequency calculations easy, but any oscillator between about 40 and 120 MHz
would work, and it could be an overtone crystal oscillator instead of one of the commercial units
(which have the advantage of simplicity.)
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GM8UPI
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« Reply #6 on: October 30, 2010, 09:09:59 AM »

OK some good ideas. Thanks.
I've managed to source a couple of SBL-1 DBM's, so will go along this route. Have got a data sheet, but to be honest, not sure how to connect as doubler!
Think I will build into small die-cast box, BNC in/out, perhaps with 2mtr band pass filter at output, if this is necessary, keeping output and input imp at 50 ohms.
Anybody help with a practical connection circuit for this ?

Dave
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WB6BYU
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« Reply #7 on: October 30, 2010, 09:54:10 AM »

The SBL-1 is easy to use, but you have to make sure that you connect all the pins because they
bring every connection out to an individual pin.  And it is physically and electrically symmetrical,
so easier to get the proper pins.

The two pins at one end of the case are the RF input.  The two pins at the other end of the case are
the LO input - they are interchangeable.  Generally one pin of each pair is grounded and the signal is
applied to other, but the can be used differentially.

The 4 pins in the middle are the connections are the IF output.  The two towards one end are
connected together and used as the IF port, the two at the other end of the case are connected
together and to ground (though again they can be used as balanced outputs.)

So generally you will use the SBL-1 with 4 pins grounded:  the RF and LO ports will be at the ends
of the case and the IF port will be two pins connected together.

For your application, apply the output of the signal generator to the LO and RF ports and take the
output from the IF port.  (And it likely will still work if you swap the ports around.)  You probably
want to use capacitive coupling on the output, as I think there will be a DC component.  (The
IF port is the only one that is DC coupled - it is sometimes used as a phase detector.)

You might check the Mini-Circuits web site to see if they have an application note on using the
DBM as a frequency doubler - don't worry if the examples use a different mixer type.

To use as a mixer, apply the oscillator to the LO port, the signal generator to the RF port, and
feed the IF port to the receiver.
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GM8UPI
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Posts: 84




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« Reply #8 on: October 30, 2010, 10:15:46 AM »

Thanks WB6BYU. Excellent, just the info I was looking for. Smiley
Do you think the BPF at O/P would be necessary ?  I believe the SBL-1 isolation figures are quite good.
As I need to get a 145 Mhz output covering the range 1uV to around 20uV, at 50 Ohms, (based on rig sensitivity figures) I will probably need to include a resistor Pi attenuator at the output.
Many thanks for all suggestions.
I will post how I get on with this>

73

Dave
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WB6BYU
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« Reply #9 on: October 30, 2010, 10:47:38 AM »

Quote from: GM8UPI
Do you think the BPF at O/P would be necessary?  I believe the SBL-1 isolation figures are quite good.

The high level feedthrough shouldn't be a problem, but the output waveform may have other harmonics.
A simple parallel LC tuned circuit across the output is probably sufficient (and would eliminate any DC
component.)  I might suggest a working Q of 1, so about 20 to 25pf and perhaps 50nH should be close.

Quote
As I need to get a 145 Mhz output covering the range 1uV to around 20uV, at 50 Ohms, (based on rig sensitivity figures) I will probably need to include a resistor Pi attenuator at the output.

Don't you have an adjustable output from your signal generator?  Just use that to set the desired output.
With the oscillator/mixer combinations we added a resistive attenuator to get 10dB total loss though the
circuit so we could use the calibrated output level on the generator and just subtract 10dB.  This was
very easy to use.  You may have to make a plot of output level vs. input level for the doubler, but once
you do you should be able to set it to any level you want (up to +0dBm or so.)  Unless, of course, your
signal generator doesn't go low enough, in which case I'd put the attenuator between the generator
and the doubler.
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GM8UPI
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Posts: 84




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« Reply #10 on: October 30, 2010, 12:39:32 PM »

In regard to adjusting the DBM level output to the RX, I was under the impression, perhaps wrongly, (reading the SBL-1 data sheet) that I would need a fairly high level of input to the DBM to work effectively, eg around 0dBm.
Assummed this may have been due to the diode forward switching characteristics etc.
Is this not the case ?

Dave
« Last Edit: October 30, 2010, 02:41:33 PM by David McAlpin » Logged
WB6BYU
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Posts: 13243




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« Reply #11 on: October 30, 2010, 03:38:10 PM »

You get minimum loss through the mixer at around 0dBm to switch the diodes, yes, but the mixer still works
at lower levels (until you don't have enough drive to switch the diodes).  So as you reduce the drive from
the signal generator the output will decrease, but not necessarily in a linear manner.  You'll have to measure
the actual output level on 2m, but I'm sure you can decrease the drive far enough to get the levels you
are looking for.

With the converter scheme we used a canned oscillator and set the drive level + output attenuator for
a total loss of 10dB, then the output level was pretty linear with the generator output.
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GM8UPI
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Posts: 84




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« Reply #12 on: October 30, 2010, 04:14:36 PM »

OK, thanks for that.
I'll wire the circuit up and do some measurements.
Thanks for all the help

73
Dave
de GM8UPI
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