Saturday, 4 June 2011

Icom PCR-1000 IF Tap (Intermediate Frequency).

As regular readers of my blog (and twitter) will know I have a Software Defined Radio, also known as an SDR.  This is a very useful bit of kit, and google will be able to explain the technical details behind how it actually works, but as a geek its really exciting stuff.

Regular readers will also know I tapped the "IF" of my Kenwood R2000  but it's not ideal, as I have to manually tune the radio - such a headache ;) and I would like something I can do from the comfort of my computer.

I knew that IF taps for my PCR-1000 were well documented but I didn't fancy "going under the hood" myself.  Well I guess impatience got the better of me last week and I went ahead and did it.  Not before using a 'scope probe and hunting out the best place for it.  Despite what it says on google and other places, I didn't tap the Q85 transistor emittor, but I followed the circuit until I came to a small 0.1uF capacitor.  Since capacitors block DC and Q85 would have a DC voltage on it, I figured why not kill two birds with one stone.  I checked with the probe and I couldn't tell any difference in signal level from before or after the capacitor.

A few notes I made to make it easier for anyone else:

You NEED to remove the control board from the unit before you drill any holes for the IF Tap.  This comes out with the screws that hold it down, and you need to CAREFULLY remove the two stiff ribbon connectors that take signals to and from the RF section (the big metal shielded bit).

You also need to remove the big metal shield :)  You don't need to make any holes in the shielding at all because where the RF goes in from the antenna socket there's a small cutout which will take RG174 or similar coax (it already has some passing through).

I grounded my coax to the same point the antenna feed is grounded (see below) but you may be able to earth it to the chassis directly on the socket.

I drilled out the hole that used to be used for the grounding bolt on the back of the radio.  This was perfectly in line with the antenna socket, and I never used it anyway.  Some people have used SMA jacks or similar but I never have any patch leads with SMAs laying around so I would end up using an adaptor anyway!

With the addition of an ExtIO DLL I found, I can now control the PCR-1000 from WinRad/WRPlus and HDSDR using "Ham Radio Deluxe". 

Pictures of the tap are below - now, a question to you experts.  Should I have grounded the RG174 coax at either the tap end or the socket end...?  Due to the position of the BNC socket I ran out of space (and shield) and there isn't anywhere convenient to put a ground connection at the tap end.  The coax IS grounded at the same place the antenna lead is grounded.

I have yet to decide if I need a buffer stage like I did with my Kenwood R-2000, but what I do want to do is modify the CCW SDR again and add what the manufactuer added to later models, which is a MOSFET pre-amp ahead of the RF input.  He said this reduces local oscillator leakage quite a bit and improves SNR/Gain figures too.  I just have to prise the information out of him as I have a lot of mosfets in stock.

Answers on a post card please.

Overview of the IF tap - this is with the sockets at the top of the page.

IF tap point.

BNC socket taking the IF to the output.

Grounding of the IF output.

No comments: