Frequency counter and frequency counter

[Andrew B]

Hello again all.

I'm just looking for a little advice regarding where to hook up a frequency meter on a Yaesu FTDX 401.
I bought an ebay frequency meter, it's an 8 digit with its own supply.

I'm a little confused as it has 2 mixers, the 1st mixer has the local osc, and the 2nd mixer has the VFO, is there anywhere I can pick up a few millivolts of a frequency that is related to the one that the receiver is tuned to?

Thanks

Andrew

[G8HQP Dave]

The quick answer is no.

The longer answer is that you can pick up the VFO and add the appropriate offset, related to the crystal frequency for the first mixer and the IF frequency.

[turretslug]

The maths of the situation is also complicated by the fact that the 1st (crystal) LO is high of the signal frequency, thus the 2nd LO (VFO) tunes lower in frequency to go higher in the band

[SiriusHardware]

It looks pretty awkward.

There is no single frequency available in both TX and RX modes (or even only one of those modes) which is directly related to the RX / TX frequency. Any method used to indicate the frequency of this unit would need to:

a) Measure the VFO frequency (available to be sampled at the 'ext VFO' connector)

b) Know the receiver I.F frequency and whether the VFO runs on the high side or the low side of the wanted frequency in order to indicate the correct I.F. offset.

So far, so good. There are a number of 'frequency meter add on' kits for analogue receivers and transceivers which have built-in selectable I.F. offsets, although you have not said if yours is one of those.

c) PLUS, in this case, it would also need to know which band was selected and alter its reading by a further band-related offset.

Only a unit capable of combining these three elements will give you a direct readout in both TX and RX modes on all bands.

Unless your frequency meter is specifically made to work with this design or range of transceivers, it is unlikely that you will be able to use it in a meaningful way.

[SiriusHardware]

Originally Posted by turretslug

The maths of the situation is also complicated by the fact that the 1st (crystal) LO is high of the signal frequency, thus the 2nd LO (VFO) tunes lower in frequency to go higher in the band

...and I hadn't even noticed that.

It could still be overcome by a custom solution which would perform the necessary inversion by subtracting the measured VFO frequency from (VFO_Frequency_Max) to obtain a value which increases proportionately as the VFO frequency decreases.

Or something like that.

And that would only work well if the VFO tuning was spectacularly linear and VFO_Frequency_Max was always reliably the same.

[Andrew B]

Thank you all so much, I had an inkling that it wouldn't be easily done, I realised after a short look through the diagram that I couldn't just hook it up to the L.O..

Still, at £8 it's another tool in my toolbox AND I can use it to modernise my Advance signal generator.

[turretslug]

Well, you had, in effect, noticed it and were one step ahead by mentioning point c)!

[Arqiva BOC]

Probably not of immediate help, but might have some useful info for you - I've recently been involved in a similar discussion about adding frequency display modules where one of the rigs had a similar architecture, albeit a 2m rig -

http://www.eham.net/ehamforum/smf/in...html#msg860355

There is a link to a design which uses a ucontroller to set the offset for the band switching, perhaps you could modify the code?

[turretslug]

These 6- and 8-digit PIC-based modules are available from a number of suppliers and at a spread of prices but seem to originate from one Chinese source. The general spec. is very appealing and the option of selecting any IF offset with a resolution of 100Hz makes them easy to interface with "traditional" gang-tuned single superhets that may have a crystal filter away from the nominal IF, as well as use as a straight counter with a trusty workshop signal genny. The above thread does though highlight that interfacing anything involving multiple conversion schemes can get at least slightly involved!

Bear in mind, though, that you don't get owt for nowt and something that wings its way across the world for as little as a tenner or less won't have a timebase that is going to usurp WWV or NPL any time soon. For this reason, I bought a 6-digit 65MHz version (and I'm not griping at a quid a digit!) but I wouldn't rely on it beyond 1kHz resolution- undoubtedly a helping hand with AR88 or Advance E2 though.

This 6-digit version has a simple linear on-board 5V reg for its 9-15V input but a peer at some of the good-res pictures out there reveals that the 8-digit (and also 6- and 8-digit VFD incarnations) use a switching regulator for efficiency (and, in the VFD case, to generate the filament supply). So, they'll need a bit more care with decoupling and screening- important anyway with "fast digits" and sensitive radios.

Using the appealingly quaint term "cymometer" helps with supplier searches