An interesting bit of junk

12th Dec 2008: I can't resist this type of stuff. This is a 'Panorama' downconverter intended for the Irish MMDS broadcast system. It receives signals in the range 2500 to 2700 MHz and converts them down to 222 to 422 MHz by mixing them with a local oscillator at 2278 MHz. Up until recently, I've been using an 'Arabsat' conveter - these have a lot of gain and not much filtering, making them prone to overload from nearby WLAN activity. They also use a DRO local oscillator which is a bit rough and drifty, making these units unsuitable for narrowband signals.  The 'Panorama' type use a crystal-locked PLL local oscillator.

After a short while using this converter with a modified satellite receiver, I decided to make use of my AR5000, which tunes up to 2.6 GHz anyway. My plan was to stop the downconverter down-converting and just use its RF stages as a mast-head pre-amp. I disabled the local oscillator by removing its supply (just a track-cut) and re-routed the signal from the RF amp to the output connector. While I was at it, I blobbed across the bandpass filter to make it wideband.  On test, it showed about 15dB of gain.

The AR5000 has a wideband IF output at 10.7 MHz, and I intended to see if I could take this signal and directly demodulate the FMTV signals. Reaching for my last remaining TBA120S (if amp and quad detector) I quickly knocked up the board below. Nothing much in it - the TBA120 with quad coil and a couple of video amps/buffers.

I had been concerned that the 10.7 MHz output from the AR5000 may be a bit narrow for an FMTV signal to fit through, but looking at the output spectrum on the spectrum analyzer it seemed to have a -6dB bandwidth of around 16MHz, just enough.

Here's the finished unit on top of the 5000. Controls are a video polarity switch and a video level pot.

And finally an off-screen photo showing the result. This is the car park of an engineering firm on an industrial estate about half a mile away.

I've always been fascinated by PLL etc, and when Huff and Puff first came along in the early 70's I decided to have a go. However due to other attractions (mainly beer and girls) I didn't tackle it until the mid 90's. The circuit I copied was a fairly complex one, but it operated in a manner that I could understand so I was able to debug it when it didn't work. After a bit of fiddling it worked well, with free 'knob' tuning and lock points every 64 Hz. Then I lost interest...

Then I stumbled across Han's Site (see links page) and started reading the Huff & Puff  reference library. Oh dear. Out came the HC CMOS box and a simple two-chip stabiliser was born. It uses a 4060 osc/divider to provide the reference signal and a74HC74 dual D-type for the detector/latch and the VFO. It refused to give a lock, and after a lot of hair-pulling I made one small change to the circuit which resulted in lovely clean locks every 1KHz.  One thing that surprised me was the amount of pull the 'varicap' LED gives!

Update: I got 50 Hz steps out of this simple circuit but it was a bit iffy, so I've moved on to a different design, also from Hans site. This is the 'Huff & Puff in practice' circuit by G3DXZ, and can be found at:

http://www.hanssummers.com/radio/huffpuff/library/ttsep96.pdf

Here's the veroboard version, which turned out to be too big to fit in my homebrew Topband rig. It's very simple - a reference oscillator (this one is on 27.277MHz), a HC4060 divider, a 74HC74 D-type, and a couple of transistors. 

The finished product. To save space, it's built  ugly-style on a plain copper board with a few 'lands' cut for the main connections. This one fits just nicely underneath the VFO. It works very well and I could sit and listen to its crystal-like note all day. Step size (at 2.4MHz VFO frequency) is 52 Hz - a 28.4 MHz ref osc would give 50Hz, but I didn't have one to hand. In operation it is almost invisible. When I sit down at the rig I press the 'setup' button, which sets the varicap voltage to half supply. After that it's just like normal - twiz the tuning knob to the required frequency and as soon as you stop twizzing the circuit pulls the VFO to the nearest 50Hz lock point, where it stays. The only odd thing about this arrangement (although it makes sense when you think about it) is that any attempt to tune very slowly results in a non-tuning VFO, as the huff & puff counteracts what it thinks is drift!

May 2009:  More action on the Huff & Puff front. I built this unit as part of an all-singing hybrid PLL synthesizer, but once I'd got it working I took it apart again! This is what happens when you don't drink enough. It's essentially the same as the one in my Topband rig, but with an extra divider to give small steps at higher frequencies. It tunes 12 to 13 MHz in 30Hz steps and is a joy to twiddle.

Sorry about the blurry picture. In the middle is a real tuning cap and the box below it contains the VFO and two buffers. The other box is the H&P stuff. That's a 5-volt reg soldered to the can.

Time to fire up the speccy! I was surprised at just how clean the output is - not a twitter until about 70dB down! The analyzer's span is 50 KHz and the resolution just 100Hz.