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Auteur Topic: Spectrum Analyzer Useful Add-On, Can be Home Constructed; comments?  (gelezen 14883 keer)
F.C. Trevor GALE
Berichten: 1

F.C. Trevor Gale, PA2TG

« Gepost op: Augustus 08, 2009, 03:03:43 »

Hello there fellow PA-Microwavers and ATVers. I am a new member of the PA-Microwaves group, my background is professionally the Communications and Man-Machine Interface techniques and technologies for the Space environment. I am also a fairly active radio amateur, PA2TG (also G8GFH), living in the Netherlands for +/- 30 years.
So I have a reasonably well-equipped workshop for my R.F. acitivities, including a lot of H-P measurement equipment including 3 spectrum analyzers based around the well-known HP-141T display frame. I have another one that is part of the radio station itself. It was during a QSO with fellow-amateur Jos, PA3ACJ, that I was made aware of this forum, and possibly the idea of making what follows into an article for publising in RadCom or an equivalent journal.

Now, what I miss with these analyzers is an interface that I can easily couple to a P.C., an interface unit with digital frequency readount, and as an extra the possibility to display the sweeps on either (a) a conventional video monitor or (b) a computer-type VGA monitor with which I can easily build up a 'record' of spectrum information. This might be from one single sweep (but not relying on the storage-phosphor tube in the 141T) or from a range of sweeps that may run for an entire evening, for example when looking at possible 'stoor-signalen' (interference sources). So I get to look at my extensive maintenance manuals for the plugins for this series of analyzer, and find that the various outputs I would need are indeed available at the back panel auxiliary connectors; also, for control over the sweep (x-axis) there is an input.
Therefore I go to work on a design that has become expanded somewhat, with ADC converters, DAC converters, a microprocessor (just a typical 68xx 8-bit chip), and the necessary digital inputs/outputs for controls, switches, display multiplexing and so on.
Basically, I take the voltage (representing tuned frequency) to an ADC which gives info to the microcomputer, the 'band' information (which gives to the microprocessor the frequency-band that is in use), the 'video' or detected vertical-axis signal which goes via an 8-bit ADC to the microprocessor (256 levels is enough for the vertical display axis, at 32 steps for each 10dB assuming 8 vertical divisions). This information is written to memory, together with a value from a 'mask' control (below which any carriers become insignificant even though they will still be displayed).
Having got a microprocessor and neccessary software running, this can be programmed to run during a sweep such that the frequency display stays on-track with the analyzer, or is latched at the first / second .... eighth carrier, or is displayed as a centre-screen value, etc. Naturally a count of carriers seen on the sweep is also available (making use of the 'mask' control). All this so far can be doe with a 6802 processor (40-dil pinout), a 13/14/16-bit ADC (40-dil), two ADC0808's, one 6821 parallel interface for the display 7-segment decoding and multiplexing, and a few LS-TTL logic chips (or one can reduce the number of these chips by using FPLD's but these also need programming). Also, one 8Kbyte EPROM (2764) and an 8Kbyte RAM memory are needed. Some (Cool 7-segment displays, a couple of pot-meters and some push-switches is really all the extras one needs; you now have a unit that can display the frequency for different conditions, user-selectable. With the analog circuity that goes in front of the ADC's, it is easy to calibrate / adjust for accuracy versus the particular analyzer you have.
Until this point, I was only concerned for myself, and therefore only for the HP-141T plus HP8555A / HP8552B plugins typically used. However, I began to think in terms of future replacements as they might have to occur, even different spectrum analyzers, and then I wondered: well, many amateurs will have different analyzers: as long as some sort of X-axis and Y-axis output is available, as they typically are even on much more modern units, then this design can still be used with some minor changes in the analog offsets / gains. As a basic unit, it already offers more than, say, the external H.P. units such as the Display-Tracking generator in terms of actual display sweep information (except that there is no tracking oscillator included yet).

Next came the question of interfacing this information with a P.C. - this can be done via USP, or Serial (COM1: / COM2:) interfacing. The 'easy' way to go is to use the 6850 (24-dil) async serial chip to use one of the COM ports on the P.C., and run 'terminal' to store the output text / numbers from the unit. It can be chosen to run straight text, EXCEL-spreadsheet-type datastream, or whatever other format you may desire.

The next thing to consider was a way of driving the sweep rather than acting as a 'slave' to the analyzers own sweep generator. In the normal 'slave' mode, you can have faster sweeps than the ADC's are able to keep up with since the ADC's have sample-and-hold analog facilities that can hold the analog information long enough for all conversions to complete, but the result will be that the information in this unit will take a few sweeps to gather as the positions that were 'missed' so far are 'filled in' by the next sweep. It's more efficient if the sweep (x-axis) is under the units' control, and this is very easy to accomplish indeed with the addition of an 8-bit latch like the 74LS374 and an 8-bit DAC with an analog amplifier to set the appropriate scanning voltage for the analyzer. This divides the x-axis into 256 segments, or points, again enough resolution for the display purposes.

Now comes another added 'extra', and that is video display of the sweep on an easily-visible standard video monitor: here, if we work in colour, we can show a typical sweep in green, with (e.g.) blue grid lines and supplementary information in red text, and it can be taken a stage further to allow a 'colour-based' decay system to provide user control of the storage characteristics of the display, not relying on any storage tube characteristics but instead using collected information in display memory.. To do all this, you need a CRT controller which contains a lot of logic associated with display functions (I have chosen the 6845), and 16Kbytes of display memory for each colour (red, green and blue). I use 2 bits per pixel per colour, giving me black plus 3 levels of each colour.
The software overhead for processing and calculating video display information 'on the fly' is quite high, and if the same cpu (the 6802 here) does this then the sweep speed that can be handled falls significantly; I chose therefor to use a second CPU to handle all this, and it has to interface with the first by some means e.g. dual-port memory etc. Instead of designig all that, I used the 68121 IPC (intelligent peripheral controller) which already has an interface with dual-port memory and the main cpu, and its own internal cpu that actually handles the video interface transparently to the main cpu.

So it is now at the stage where the most important functions are all available: I have designed and built the boards so far (a one-off) by hand wiring (by soldering 0.4mm wire-wrap-wire, teflon-coated) on miniature matrix-board and have a digital board that is about 3 Eurocards in size, just a bit smaller, and one analog board of a eurocard size that goes with it. Power supplies are simple: +5Vdc for the logic Vcc, +12V and -12V for the analog circuity and ADC''s. It is not built into a box yet, but it has already shown that it will be useful as an additional tool to an existing spectrum analyzer.

My question: does this sound useful to any others out there? Have you any comments on it? Please let me know: e-mail is  . If there is interest, I'll go into furthe detail, and/or provide some photographs of the boards etc.

Thanks for reading so far - best 73;s, F.C. Trevor Gale, PA2TG.
Berichten: 39

« Antwoord #1 Gepost op: Februari 01, 2011, 02:41:34 »

Hi Trevor,

Ever looked here:

I've build it and got it working but it is not thát handy so there might be some redesign possible. The designer is PA3ESH and is quite helpfull. Drop me a line, 73!

Bas de PH0BAS.
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