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 Post subject: TROUBLE
PostPosted: Tue Dec 11, 2012 4:23 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
While testing the control signals for the off-board PA, I accidentally shorted TX_6 to 13V. Avoid that mistake at all cost!

TX_6 connects directly to the demux (U10) and to SW1. Surprisingly, the CMOS demux survived 13V. Apparently the GaAs switches did not. I replaced SW1; no help. Apparently the 13V reported through SW1 to 5VA and to the other switches. So, I replaced SW2, SW3, and SW4.

Now I can receive the 25MHz clock--somewhat. When I tune past 25MHz, the speaker background noise hushes. There are no tones, just noise or no noise. Strong SSB signals sound like weak, intermittent pulses of noise.

The AF amp works fine. AF injected at DACB makes a nice loud note in the speaker.

The LNA works fine. RF injected at the antenna terminal produces a strong, clean signal at Pin 1 of MX1.

LO1 (O1) works fine. It produces a nice strong clean signal at Pin 8 of MX1.

Output from MX! (Pins 3 and 4) is weak (a couple of mV) and very noisy. I've ordered another SBL-1; I assume it was fried, too.

I measure the same weak noise at every point I've checked through the IF chain.

Injecting 10.7 MHz anywhere along the IF chain produces the same result as the 25MHz clock signal. As I sweep thyrough 10.7 MHz, the speaker background noise hushes. No tone is produced.

Maybe the LO2 (U4) was fried, too. At C90, I measure a very weak (about 1 mV) and very noisy signal with a detectable 10 MHz component.

Can anyone tell me, with a strong RF signal (5mV) at the antenna, what approximate signal levels are expected at:
The output (Pins3 and 4) of MX1?
And at various points through the IF chain (say at C82, C80, and c78)?

What signal level is the LO2 (U4) expected to produce (at C90)?

Any ideas/suggestions?

Thanks in advance.


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 Post subject: Re: TROUBLE
PostPosted: Tue Dec 11, 2012 9:54 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
The LNA's have 20ish dB gain so that will be more gain then the filters and mixer have loss. So with 5 mV in you should see roughly 50 mV out of the LNA and maybe 30 or 40 into the mixer so 15-20 into the IF filter and maybe 8 to 12 into the IF amp.

What does the signal into the A/D look like when the hushing happens, it is railed? or no signal at all? The OP amps run of the 5V line maybe they are toasted too? Try injecting a signal at the 2nd IF 16.8 kHz and see what happens.

Jim.


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 Post subject: Re: TROUBLE
PostPosted: Wed Dec 12, 2012 7:19 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
I found, and fixed, one small problem this afternoon. I'm not sure if it was a result of the 13V surge. Maybe it's an older problem that might have contributed to the low gain/AGC problem that we have discussed under the AGC forum topic. L3 was defective. I replaced it and the signal strength at the RF input to MX1 increased from about 1.5 mV to 3 mV.

Here are the signal levels I'm measuring now.
RF injected at antenna input 5 mV
LNA output 80 mV
MX1, RF in 3 mV
MX1, LO in 140 mV
MX1, out 2.5 mV
IF, 1st stage out (at C80) 26 mV
IF, 2nd stage out (at C76) 650 mV
LO2 out (at C90) 13 mV (very noisy)
2nd mixer out (at C81) 16 mV (very noisy)
DSPic, AN0 input 1.58 VDC with about 30 mV AC superimposed
DSPic, AN1 input 1.53 VDC with about 30 mV AC superimposed

The superimposed AC level on DCPic AN0 and AN1 does not change significantly as the receiver is tuned above, below, or through the signal generator injection frequency.
However the audio level changes from normal low level audio noise when tuned well above or well below the injection frequency, to almost complete silence when tuning through a 24 KHz wide band segment centered on the injection frequency.

My el cheapo signal generator is limited to frequencies above 100KHz; so I can't inject at the second IF frequency.

I appreciate you help Jim.


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 Post subject: Re: TROUBLE
PostPosted: Wed Dec 12, 2012 9:34 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
Correction to my last post. I was confusing AN0 and AN1.

The AC superimposed on AN1 is almost 0 mV. As you tune the receiver upward and approach the injected frequency, audio noise and AC increase to about 17 mV at the center of a narrow peak, then the audio becomes almost silent and AC decreases to 0 mV through a band width of about 24 KHz, then audio noise and AC increase to another peak of about 25 mV, then back to normal low-level background noise. The 2 narrow AC peaks are separated by about 28 KHz. They are not symmetrical; the upper peak is significantly larger than the lower frequency peak.

AN0 does not change.


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 Post subject: Re: TROUBLE
PostPosted: Wed Dec 12, 2012 9:48 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
So there's something going on in the front-end 80 mv to 3 mv is like 30 dB loss between the output of the LNA and the input to the mixer. Even if the filters had an injection loss of 6 dB there should be 40 mv so something is going on.

A noisy 2nd LO is also going to translate to noise in the 2nd IF so see what's up there.

It would be nice to know if the 2nd IF and DSP are working can you download a soundcard siggen?

Try this one...

http://www.softpedia.com/get/Multimedia ... CGen.shtml


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 Post subject: Re: TROUBLE
PostPosted: Thu Dec 13, 2012 8:39 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
I appreciate the sound card siggen idea Jim. I downloaded WSCGEN.EXE, injected 16.8 KHz at C6, and observed a crisp, clean tone from the speaker. Clearly the IF2, DSP, and audio stages work fine.

I made a few more measurements around the LO2; and found that the output of U4 (pin 10) is railed at 5VDC (plus some low-level AC noise). A replacement AD9833 is on order.

As for the filter between the LNA and the 1st mixer, I desoldered and remounted all the components, and replaced a couple of parts that I had spares for. Got about 3 dB of improvement. The RF input level to the 1st mixer is up to 7 mV now. That's about all I can do to it for now, until I get replacements for the other parts.


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 Post subject: Re: TROUBLE
PostPosted: Thu Dec 13, 2012 11:23 pm 
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Joined: Mon Jul 23, 2012 6:25 am
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Go part by part from the output of the LNA to the mixer, there has to be some bad component. We need gain before the mixer!


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 Post subject: Re: TROUBLE
PostPosted: Fri Dec 14, 2012 9:11 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
I replaced the AD9833 this evening. Now I'm receiving the 25MHz clock normally!!! :D

And apparently my tinkering with the filter between the LNA and the 1st mixer improved gain enough that now the AGC responds to strong signals.

So, all together, my clumsy error of shorting 13V to TX_6 fried the GaAs switches and the AD9833 in the receiving stages. (I think the filter problem predated the 13V short.)

I haven't tried the transmitter yet. (My fingers are crossed.)


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 Post subject: Re: TROUBLE
PostPosted: Fri Dec 14, 2012 10:01 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
You are the man! Tackling a project like the DSP-610 takes some serious troubleshooting and tracking skills.

Jim


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 Post subject: Re: TROUBLE
PostPosted: Tue Dec 18, 2012 7:44 pm 
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Joined: Fri Aug 03, 2012 5:35 pm
Posts: 36
Well, I tried the transmitter--no go.

Found that U5, the double balanced mixer, was toasted, too. Replaced it.

Now, along the transmit chain, with the transmitted keyed, in SSB mode, with no audio input, I measure the following.
U5, pin 6 (LO2 input) 80 mV
U5, pin1 (DSP_TX input) 0 to 3 mV, depending upon setting of VR1 (DSP_TX is 60 mV at C89, before the voltage divider)
IF_TX at C104, 6 to 21 mV, depending upon setting of VR1
IF_TX at C58, 6 to 20 mV, depending upon setting of VR1
IF_TX at MX1, pins 3 and 4, 3 to 14 mV, depending upon setting of VR1
RF, at MX1 pin1, 0 to 7 mV, depending upon setting of VR1
RF at C45, 0 to almost undetectable

Clearly, I'm still loosing lots of signal strength through the filter between SW3 and MX1; but the signal strengths seem rather low from U5 forward.
What do you think Jim?


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