New expansion - uPD2824 chassis

[cb4ever104]

Stage two - broadband the radio

Hi Sue .

I'm thinking of having a go at doing the RX and TX tuning mods on one of my 2824 radios . I'm going to be using through hole components , so I just wanted to check a few things . Before I start I'll check your RX and TX pcb layout is the same as mine , as we had some issues with this when doing the expansion part .

1) You say that you replaced the original VCO varicap with a SMD . What equivalent through hole part # would that be ?

2) I'll check the RX "trackwork" that you did against my radio .

3) Looking at the RX and TX parts , I think I'll need 5 x BB809 , 1 x 0.1uF ceramic , 2 x 0.001uF ceramics on the RX and 3 x 100k resistors . Can you just confirm what value the cap is on the TX board . 0.001uF ?

4) You started with 3 diodes on the RX test , but removed one . Right ? So I now only need 4 , and not 5 . Right ?

5) I need to replace C77 with a 56pF .

I think that's right . Could you just double check it for me .

Thanks !

[lbcomms]

1) Same as the mixer stage tuning diodes, BB809 (thru hole ones have a yellow band).

3) I used 1nF (i.e. .001uF or 1000pF) but it's not critical. It's in series (RF wise) with the 50pF varactor <do the capacitors in series math to see why it's not critical at all>, so anything from 300pF upwards will do. Don't go way above that though (such as 1uF) because inductance will start to mess things up...

4 and 5) There are three circuits (the VCO, Tx mixer, and Rx mixer) that need to track / sync each other, for example if the VCO tuning voltage is 2V at 26 MHz and 6V AT 28 MHz then you want the mixers to peak at the approximately the same voltage for the frequency it's operating at.

Temporarily replace the PLL with a pot (and disable the lock detector) rather than trying to do the channel mods first.

Draw the response of each of the three in different colors on a basic graph - vertical (Y) scale the voltages, horizontal scale the frequency.

The number of varactors and the size of the VCO cap (C77 in our example) determine how "steep" the the slope is, so experiment with the VCO cap value and the number of mixer varactors to get them as close to each other as possible. The Tx mixer needs 2 as it's balanced, so plot that one first as it can't be altered. Then do the VCO cap to get it to track as close as possible, and then do the receive mixer ones.

With a bit of patience, you can get these radios to cover at least 2.5 to 3 MHz with good performance.

[cb4ever104]

Many thanks Sue. I'll start to get my head around it next week.

Regards