[DSP-10] CW transmit has a raspy sound

[Bob Larkin]

Hi Lee,

I must confess to being somewhat overwhelmed!

Can we focus for a moment on one item.  How about your own DSP-10?  Picture 
#6 of the 124 MHz pix (I will call it 124-6) shows a reasonable VCO 
spectrum, free running. Pix 124-10 shows that osc (with a transistor 
change) locked and it is terrible.  To me, this says it is a PLL problem, i.e.
* The component values in the loop filter.
* Noise on the 10 MHz reference.  <-  Unlikely as per your tests.
* Noise on the power supply coming from U103
* The wave form of the signal coming back to the PLL from R112

But, not a VCO problem, is it? The PLL should reduce the close in noise on 
the VCO.

The noise modulation required to mess up the phase is in the microvolt 
range and can't be measured directly. We must deduce its level from the PLL 
spectrum.

But, before trying to suggest more avenues, is this the right problem 
description---your DSP-10 gets noisy when the loop is closed?

Lots of fun!!

73, Bob

At 05:55 PM 3/31/2006 -0500, you wrote:
>Response below
>
>At 10:42 AM 3/30/2006 -0800, you wrote:
>>Hi to All!
>>
>>Lee, the SA photos that you posted with C113 shorted, look OK. These look 
>>to be well within the range that will be cleaned up by the PLL. They look 
>>comparable to what I see and mine cleans up fine with the PLL!
>>
>>The level's of the LO at the TUF-1 is fine. I believe it may be a few dB 
>>over the +7 dBm nominal, but this should be OK. If one wanted to reduce 
>>this a bit, the DC power feed resistors to U106 would be worth playing 
>>with, but i doubt that one could see any difference afterwards.
>>
>>So, the problem would seem to be with the PLL. Possible areas might be:
>>* The component values in the loop filter.
>>* Noise on the 10 MHz reference.
>>* Noise on the power supply coming from U103
>>* The wave form of the signal coming back to the PLL from R112
>>
>>Let me add a couple of words on the latter item.  I originally did not 
>>have C146 in the circuit, and would see noise on the PLL, not unlike what 
>>you see. Experimentally, C146 provided a major change, even though it has 
>>little effect on the 126 MHz level. C125 and C146 are separated 
>>physically by perhaps 35 mm and act as a LPF on that line. I speculate 
>>that a sine wave makes the LMX1501 happier than a distorted wave form. 
>>You might try adding a small cap of a few pF across C146.
>>
>>Good Luck!
>>
>>73, Bob  W7PUA
>
>Here is the latest.
>
>Prior to Bob's reply, I started making a test oscillator essentially 
>duplicating the original components but as a free running osc and no 
>PLL.  The output was a fairly clean signal at +12dBm. It is in fact, much 
>cleaner than the 124MHz signal with the PLL locked on the DSP-10.  The 
>only difference between this and the DSP-10 is the J310 I used was an 
>SOT-23 device and I had adjusted the values of C117 and C118.
>
>I replaced the J310 on my DSP-10 with an SOT device and the signal cleaned 
>up a bit when compared with the same setup prior.  I would say I reduced 
>the noise at least 10dB if not more.
>
>I than tackled the overdrive question of U105.  I replaced the 3.3pf with 
>a 2.2pf cap and saw no reduction in the output to speak of.  I then 
>reduced it to a 1.5pf with a total output reduction of 0.5dB.  I than put 
>the 3.3pf back and placed a 10pf to ground on the input to U105.  This 
>reduced the output of U106 to +11dBm.  There is a slight crook in the wave 
>form on the output of U105 and this reduced the crook a bit.  It is 
>beginning to look more like a sine wave.  I see nothing wrong with the 
>output of U106 as it looks like a perfect sinewave.
>
>The output of the J310 is about 1v P-P and the input to U105 is now at 
>150mv P-P. The output of U105 is at 800mv P-P and the output of U106 is at 
>1v P-P. This is with the SA connected so it is into a 50 ohm load.
>
>I then unshorted C113 and R152 to enable the PLL.  I booted the DSP 
>processor and the resulting SA waveform did not look good at all. (in face 
>a bit worse to say the least). I then placed a 6.8pf cap from the Fin line 
>to ground at the point where the line via's to the bottom of the board. No 
>real help. I also tried temporarily setting a 6.8pf on top of C146 and 
>this made no discernable change .
>
>The waveform on the Fin line at R122 looks identical to the output of 
>U105.  At C146, the signal has degraded and was somewhat improved by the 
>additional cap in prev paragraph.
>
>I did check the values of the components in the loop filter. Nothing wrong 
>there.  Maybe a bad component?
>
>The 10MHz osc looks like a nice clean sinewave with the internal osc and 
>like a messed up digital waveform with my N1JEZ GPS locked osc but 
>switching between the two makes no difference.
>
>The little noise I can see (10mv at a freq higher than 10MHz) on the 
>output of U103 appears on the ground lines as well.  It's just scope probe 
>pickup.  I can see no low frequency noise.
>
>I've added a few photos to the bottom of the web page.
>
>http://www.aa1yn.com/dsp-10/spec_ana/
>
>
>
>
>Lee Scott - AA1YN http://www.aa1yn.com
>Home of the VHF & Up Register http://www.aa1yn.com/vhf
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