Frequency synthesizer for Pye Westminster 4m band

[unitelex]

Hoping somebody has experience with adding a frequency synth to a Westminster FM model.
I understand the principles of the two dividers and phase comparator
but not sure about a few aspects
For example VCO design.
Whether it s better that the VCO is operating at the carrier frequency or better to inject the VCO at a lower frequency into the multipliers.

I have seen a few commercial frequency synth offerings, would be good to know if somebody already had one of these working well on a Westminster ang hopefully pick up some tips/recommendations.

Thanks
Chris

[Sean Williams]

Tom Miller G4BYE did this some time ago. Might be worth looking him up

[timewave]

I did it with a 2M F25FM base station, which is just a W15FM with a mains PSU and a bit more RF output, back in the 80's. I used a Wood and Douglas kit and I seem to recall that the TX synth output was in the 4MHz region and was injected into the first multiplier. I think the PLL line was modulated.

I loaned the whole thing to a local G8 but never got it back.

John G4IJD

[Radio Wrangler]

The Tx side ran witha rather low crystal filter and plenty of multipliers so that a modest amount of FM could be introduced at 4MHz and the multiplication process brought it up tothe wanted amount of deviation.

With a switched crystal channelised radio trying to FM a VCXO gives different amounts of deviation on each channel because of crystal differences... so the designers preferred to use a phase-modulator stage to apply a small amount of FM. This made it independent of crystal parameters and also an awful lot more suitable for adding a synthesiser.

Because of the large amount of multiplication for Armstrong method FM,the step size of the synth needs to be rather small so that means either a high frequency synth with a divider on the output, or else using one of the many DDS kits on the go (Usually AD9850 chips.)

You'll need a different arrangement for receive.

David

[unitelex]

Thanks for all the information and tips.

Receive side
I think I will do the Receiver first.
The original RX xtal is 3rd overtone and it is followed by a x2 multiplier
fx= (Fc-10.7)/2

so I guess I could inject ~29 to 30MHz into the x2 multiplier, or even re-tune it as a x3 or more? and inject ~20 Mhz etc.

Basic question about the phase comparator: the output from a divide by N is not a square wave, does that matter? I'm assuming I have to add a divide by 2 to get a square wave for the phase detector?

TX side
I take on board the comments on the value of the phase modulator. So I prefer to keep the original W15FM phase modulator in use.
The TX xtal multiplier is x24, so I need to inject ~3MHz

Ideally I'd want one DDS chip that can do RX and TX. Could the AD9850 do it? Would i need separate VCOs for RX and TX or could one VCO switch between RX and TX?

[Restoration73]

Cirkit did a board many years back for this one:
https://www.nxp.com/files/rf_if/doc/...MC145151-2.pdf

[Radio Wrangler]

It depends on what sort of phase detector.

A very very simple one is nothing more than a diode ring mixer or an exclusive-OR gate. These do benefit from squarewaves. But they are only phase detectors. More commonly PLL ICs use phase-frequency detectors which are a couple of flip flops with or-gated common feedback. These trigger on edges and don't care about pulse widths. These detectors, if out of lock give a DC level saying whether the VCO needs to be slewed up or down to acquire lock.

The AD9850 DDS can be clocked up to 125MHz, and so the output can be, in theory, used up to half of that. 62.5 MHz. But that requires an impossible filter. Up to 30MHz is quite feasible and there are kits for this.

With the PMR set multiplying things so much, it will have the effect of exaggerating the DS spurs rather a lot. So I'd stick to running the DDS in the 30MHz region and dividing its output down to 3MHz. The difference in spurii from these two approaches is about 20dB.

A DDS doesn't need a VCO or a PSD. It computes a sampled sinewave at the required frequency then a DAC and lowpass filter reconstruct it.

David

[pastyboy]

I used to run what was called the "fresh design" dual gate mosfet vco running on band 2 very simple and reliable, used to run on b1 also and there is a range of rohm chips which will synth it at operating frequency. Some had built in stereo encoders too. But we won't bee needing that function. You could easily make the IF shift.

[unitelex]

I have used MC145157 in the past, not for radio, it was for a special baud rate generator.
I thought that series was now obsolete but maybe not... its frequency was limited to 30MHz IIRC

Sounds like the AD9850 is the way to go. There are some assembled kits available with 2 line LCD, and including AVR micro and some demo software to set up frequency output. It looks like there is no source code with these so would have to write S/W from scratch to get a decent UI that allows stepping through channels and computing the RX and TX frequencies

Good advice about running it at 30MHz, and dividing down to 3MHz (for TX)
For RX, should I divide down to ~3MHz then multiply up again rather than injecting the 30MHz into the W15FM x2 multiplier?

[Radio Wrangler]

No run the DDS at 30 ish MHz on receive so you don't need to multiply up as much.

The only reason for going down to 3MHz was for the phase modulator which can only make a small amount of modulation and needs ahigh multiplication factor to scale it up.

David

[unitelex]

OK got it, thanks for all the advice