Friday 24 January 2014

Plumbing in the Filters

Having made and tested my new I2C-controlled filters, it was time to plumb them into the rig and go multi-band...

I made up some little patch leads from RG58, so I could plug the filters into the sockets that held the plug-in filters. That way - I supposed - I wouldn't need to make any modifications to the rig to incorporate the new filters. I should be so lucky!

 Here is the rig with the new filter boards patched into the signal chain...

On trying the filters, I noticed an immediate problem on 80m. As soon as I pressed the PTT or the key to put the rig in Transmit, the LowPass filter board tripped out - none of the filters was selected and no amount of subsequent band changing (thereby forcing my ATmega328P to send new commands to the MCP23008 IO Expander) would rescue things. I needed to "power cycle" - the new name for turning things off and back on again, for those who prefer to call a spade a personal, horticultural entrenching tool.

This was fixed pretty quickly by adding some more decoupling to the IO Expander chip - I already had 470nF about 2cm from the chip, but another 100nF right near the VDD pin and similar on the collectors of the relay drivers (on the 80m band only) soon cured things.

This fixed the LPF. However, plumbing in the new BPF was causing much greater problems...

Everything worked FB with the rig driving a dummy load. However, when I worked into my antenna system, I was seeing all sorts of problems on the meter on my tuner.

Looking at the signals on the oscilloscope soon identified the problem - the rig was generating out-of-band signals (at LOWER frequency than the intended RF). This was causing the indicated SWR to be very poor (which it was not; the antenna was correctly tuned for the band in question, but NOT for the additional lower-frequency content).

Eventually, I figured that this was caused by some kind of feedback path from the antenna back into the rig - which was sitting there naked on the bench, without shielding its modesty in a nice enclosure. With this differential diagnosis in mind, I set about looking for the source of my woes.

The out-of-band stuff was bad when I spoke into the rig and was strongly triggered by low frequency content in my voice. Conversely, a high frequency whistle seemed to provoke much less out-of-band output. This made me wonder if the feedback was somehow getting into the AF path - but careful investigation and measurement showed it was contained within the final stages of the transmitter.

Eventually, I discovered that things were made very much better if I removed one particular segment of wire on transmit, seen here labelled accordingly...

The offending wire (in fact, a length of coax, some 10cm long) was driven at one end by the PA output during transmit, BUT WAS OPEN CIRCUIT AT THE OTHER END, due to the action of one of the Transmit / receive relays. 

Here's the relevant section of the schematic...

I decided that the quickest path to a solution would be to add another little relay close to the final transistor, in series with the one already there, such that the little piece of wire is automatically removed whenever the rig is in transmit. Here's the plan...

And here's the actual fix on the rig...

Strangest thing of all - I'd been supported and encouraged through some of this fight with the RF feedback by my friend, mentor and collaborator (watch this space!), Pete Juliano, n6qw. Turns out that he's had a VERY similar problem in his build of a BITX20 - which he solved by adding an extra relay too.

Now, the rig works on all four bands, without having to do anything more than drive the menu system on my "VFO". Of course, I still have to adjust my antenna and (of course) the g5rv is still little more than a dummy load on 17m but at least the rig is now genuinely multi-band.

As I admitted before, things are not yet "perfect"; I still see faint traces of the original problem (which TOTALLY disappear when firing into the dummy load) but I'm not going to do any more detective work to improve matters until the rig gets a properly shielded enclosure. It is entirely usable now.

Instead of wasting time chasing perfection, I'm off to explore four different bands at the "flick of a switch"!

...-.- de m0xpd

1 comment:

  1. I'm glad that you were bale to fix the problem by using the decoupling. It can sometimes take hours to troubleshoot a circuit if you keep missing whatever is going wrong. At the same time, sometimes a fix is obvious but you don't have the equipment you need to implement a workable solution. Thanks for sharing this project with your readers.

    Traci Romero @ Harris Plumbing