[Lowfer] Genesis G500 Update

[n8tl]

Latest info for 600 meter SDR is given below:

Tom N8TL

9a. Genesis G500 RX on air !
    Posted by: "nick" nick at clockmaker.com.au vk1aa
    Date: Thu Dec 16, 2010 3:59 am ((PST))

Hi  all -

this is a VERY first screen shot of G500 SDR transceiver, RX assembly only.
The receiver coverage is 410 KHz to 590 Khz with 192K sampling rate.
(note the frequency readout is actually +500 KHz higher because software needs to be updated)
Like with the 137KHz version, the initial results look promising.
Visible on the left are two local beacons, BK-416 and GLF-428 . 
In the middle is trace of my AX2VKX beacon. And the monster signal on the right is
ABC Sydney 2RN at 576 KHz. This AM signal is S9+55 dB here.
I was able to reject image in hardware by -55dB and further -40 is achieved in software
which is really good for a simple kit using thorough hole components.
This is almost 100dB of image rejection!  Antenna is Mini Whip.

Of course, the purpose of this transceiver is not to cover the entire 200KHz but
only 20-30 Khz bandwidth just above the 500KHz but nevertheless, wider receiver could be useful for band monitoring.

Hate to make predictions, but if all goes well, the transmitter side should be ready and running by the New Year eve :-).



BTW I had a problem with assembly / 4066 mixers. Somehow I've managed to install top 4066 in wrong holes!
It sounds crazy but there are two ground holes (vias) just next to the IC, with exactly the same pitch.
What a booby trap that is! Anyway, I've noticed the mistake while soldering the last pin so
ic was removed and new one soldered properly. This is the green G40 PCB.
Upon the first test with Winrad I've oticed that I could not get the image rejected more than -22dB and
that was with trimmer way up (total resistance 192 Ohm). 
Funny enough, with GSDR, the image was very good thanks to WBIR, but not good enough to 
reject all traces of that monster AM transmitter.
So went back to Winrad.  Noticed that level of one channel is 10dB higher than the other which was most likely reason for poor
rejection.This can be easily observed in Winrad  Show options->Mode-> left (or  Right) ch only.
(Goran: maybe we can have that option in GSDR too? It is very handy!)
Anyway closer inspetion of pcb reviled the culprit: it turns out that I managed to shortcut two pins on 4066 with bit of trace which come off the pcb during de-soldering.
Installation of new mixer chip restored the level of  I and Q and  image was rejected in hardware by -55dB.
The troubleshooting tip: measure the voltage on each 33R resistor after mixers. It should read 0.00V or very close to that
value (power on, no antenna connected). In my case I had zero V on three resistors and 0.017V on the fourth
so that was a dead giveaway :-)

Bottom line: don't talk on the phone while soldering ICs! And 10dB difference in signaln levels will result in 30dB poorer image rejection.


73 Nick VK2DX  




Messages in this topic (2)
________________________________________________________________________
9b. Re: Genesis G500 RX on air !
    Posted by: "nick" nick at clockmaker.com.au vk1aa
    Date: Thu Dec 16, 2010 4:12 am ((PST))

Forgot to include this screen shot:
hardware image rejection of G500 after mixer replacement. Actually  510 KHz transmitter was pushing 7W into dummy load so
this is the strongest signal I could generate. The image is not even detectable!



73 Nick



-- 
n8tl <n8tl at woh.rr.com>