40 Metre Spectrum Analyzer

Why 40 metres? Well, I have a dipole for it strung down the side of my home unit and there are lots of broadcast power-houses just above the HAM band that my very poor antenna picks up pretty well. Also because I was inspired by JF1OZL's unit, which is also for 40 metres.

My circuit is quite different in implementation, but is identical in high-level concept. A swept VFO LO using a 555 timebase mixes up the RF to a higher crystal filtered IF, which is then amplified and detected, the "DC" being fed to the CRO Y-channel.

Spectrum Analyser

I built a similar VFO to JF1OZL, but I added a buffer to improve stability and drive capability. My timebase is essentially identical. My crystal filter is shorter, only 4 crystals at 11.98 MHz, and my IF amplifier is completely different. I did not have the TV IF chip that JF1OZL uses, so I built a two-stage RF amplifier using BF199s and ferrite bead collector 9:1 transformers.

The front-end JF1OZL uses I built (using a 2N5484), but had many troubles making it stable. I tried using a J310 and backing off the gain, eventually getting it to behave. It is nice and broad-banded, but I think a low-Q dual coupled-resonator front-end would be better, followed by a broad-ish amp, this would also make the unit easier to band-switch - I have ambitions of making it multi-band eventually. The current front-end works, but I am still unhappy with it. One day I'll rebuild it and see if I can improve it.

The device works as advertised. It is very interesting to watch 100+ kHz of band at once. The broadcast-alley part of the band is very alive at night, and you can see fade regions moving up and down the band. Exactly how this works in the ionosphere I don't know - must research that.

My poor antenna makes 7.0-7.1 Mhz viewing a bit disappointing. However, it is still very useful to watch CW signals jumping up and down, and SSB QSOs pushing up out of the noise now and then when then band is open.

This unit on 20 metres would be even more useful, and perhaps on 80, 10 and 6 too. A simple digital decade counter could cycle-through the bands periodically, switch the clock signal to implement a "hold/go" control, and perhaps a manual "advance" button to cycle through the bands manually. That way it could watch all your bands of interest continuously, outputting to a junk AF CRO or perhaps a soundcard with some software that looks for interesting signals (and/or logs the entire day to a database with a web interface!).

No circuit diagram yet, for now you can mostly work it out from the picture (huge version linked below).

Spectrum Pictures

It is quite difficult to photograph the CRO display with a digital camera. Your eyes are slow enough to see the image nicely, but the camera shutter is too quick and it gets images like this:

Short Exposure (partial sweep)

However, with a large increase in the sweep rate, I managed to capture a full sweep. Note that the selectivity has been compromised by sweeping at at much higher rate than is needed for a good eye-compatible display:

Short Exposure and Fast Sweep (poor selectivity)

Playing with the exposure settings on the camera and holding it still I successfully captured a shot of the broadcast band above 40 metres at a sweep-rate that also gave good selectivity:

Longer Exposure

I've had to darken this image in post-processing, as it was over-exposed (you can no longer see the image on my notepad for example). Note the huge signals seen in comparison to fairly weak ones. Those little blips aren't intermod either, they are real stations that fade in and out.

If you disable the sweep and manually tune across the band with the VFO trimmer you can receive these AM stations using the spectrum analyser as a normal super-heterodyne receiver. A small amplified speaker connected to the Y-channel output works quite well. The recovered audio is actually pretty good, the general circuit makes a good shortwave radio (there is no AGC of course). I may add this "feature" to the completed unit when I build it into a box, some way of turning the sweep off and tuning manually with a pot, and of course a small audio amplifier.

Note the retrace problem. The output at pin 3 of the 555 is in-phase with the sweep, a simple diode or transistor switch for blanking will definitely be implemented in the completed unit. The unit could use more shielding, its amazing that it works as well as it does with the primitive lash-up, that can't be offering a very good ground. It is probably only the miracle of distributed gain that the superhet gives you that lets it work as is currently is.

A LF log-amp or log RF detector would make this unit capable of real dB scale measurements. It is really a true spectrum analyser, as simple and crude as it may seem, it is a useful device for narrow-band monitoring.

Update: 2007-01-28

I've started boxing up the unit. I found a nice cabinet in the junk box and started looking for sockets, switches, pots, knobs, etc. Unfortunately I did not have the smaller knobs required for this fairly small case, so I didn't complete the unit, but I did install it completely in the sense it now performs identically to the original lash-up, and everything is terminated nicely to the back panel.

Initial Boxing-Up

Yet to be done is adding the pots to control the sweep width and centre frequency, and the switches to disable sweeping and use the radio as a shortwave receiver. You'll note I've left room at the front for an AF power amplifier, and the entire front panel is yet to be drilled. I'll probably put a headphone socket right on the front panel.

1 comment.


title type size
Larger Picture of Spectrum Analyser image/jpeg 156.532 kbytes