Boat Anchor Receivers - AM Audio S/N Measurements

[W1VD]

Steve

Yes...but all is not lost.

Just hung a scope on the AGC line. Sure enough, there's substantial audio riding on the line  and, as expected, it gets worse the lower the audio frequency. It's about 8X worse at 100 Hz as compared with 1 kHz.

As a quick test to see whether this was the cause of the higher distortion a .1 uF was bridged across the existing .01 uF main AGC capacitor. Results are as follows:

Receiver: National HRO-60 stock AGC

Mod	100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz
30%	5.0%	3.5%	2.8%	2.5%	2.2%	2.2%	2.8%
50%	7.1%	4.0%	2.5%	2.0%	2.0%	1.8%	1.8%
70%	7.9%	4.5%	2.8%	2.2%	2.0%	1.8%	1.6%
90%	8.9%	5.0%	3.2%	2.5%	2.5%	2.2%	2.2%
100%	11.2%	7.9%	5.6%	4.5%	3.5%	2.8%	1.8%

Receiver: National HRO-60 modified AGC

Mod	100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz
30%	3.2%	3.2%	2.5%	2.5%	2.5%	2.5%	2.8%
50%	2.8%	2.0%	2.0%	2.0%	1.8%	1.8%	2.0%
70%	2.5%	2.0%	2.0%	2.0%	2.0%	1.8%	1.8%
90%	2.5%	2.0%	2.0%	2.0%	2.0%	2.2%	1.8%
100%	5.0%	4.5%	4.5%	4.0%	3.2%	2.5%	1.6%

Again, this was just a quick test to prove that the AGC was a contributing factor to the problem - not to provide the best possible cure. A previous thread http://amfone.net/Amforum/index.php?topic=12441.0 from Steve WA1QIX addressed the ills of AGC and detector stages in tube AM receivers. Steve attacked both the AGC problem as well as the detector's ability to cleanly demodulate modulation levels in excess of 100%...all as an add on 'module' to an existing receiver. Looks like I'll be building one in the near future...

Suspect I'll be seeing more AGC problems in other BA receivers undergoing testing.

       

[WA1GFZ]

Gee I wonder if a series diode would help to peak charge the AGC delay cap

[Steve - WB3HUZ]

Yep. The AGC on most of these receivers was just not designed to deal with low frequency modulation, let alone high modulation levels at low freqs.

[w3jn]

Note the difference in distortion measurements I saw in the R-8 slow vs. fast AVC.

Probably the cleanest sounding RX I have is a WJ-8716.  But put it on FAST AGC and it sounds terrible.  This problem isn't limited to BA receivers.

[Steve - WB3HUZ]

Fo sho. That's why I cringe when I see people using their R-390A with the AGC set to Med or Fast for AM.

[WA1GFZ]

John,
The 8716 doesn't have anything special for AM detector so I bet the AGC is set up properly. That RX, designed in the 70s was ahead of its time.

[W1VD]

Some additional numbers to chew on...

While measuring receiver distortion it's a simple matter to look at recovered AM audio bandwidth. Using the 1 mV rf, 30% modulated signal it's a simple matter to step through the audio input frequencies and note the level of the detected audio. The results will be related to i-f bandwidth / shape factor, audio tone control (if any) and overall  audio stage flatness.

Some of the BA receivers being tested, like the HRO-60, have a convenient external audio input which makes testing of just the audio amplifier simple. Many BA receivers have audio tone controls that provide variations of low cut, high cut, peak and flat response. The HRO-60 manual shows three curves for three different setups. Actual measurements with these setups track reasonably close to the graph shown in the manual and attachment #1 below. Before making the recovered AM audio bandwidth test the audio response was made as flat as possible (within reason) using the adjustable tone control. Results are as follows:     

100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz	3 kHz	4 kHz	5 kHz	6 kHz
+1 dB	+1 dB	0 dB	0 dB	0 dB	0 dB	+1 dB	+2 dB	+1 dB	+1 dB	0 dB

Results of the recovered AM audio bandwidth for the widest i-f filter are as follows:

100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz	3 kHz	4 kHz	5 kHz	6 kHz
+2 dB	+2 dB	+2 dB	+1 dB	0 dB	0 dB	-5 dB	-16 dB	-32 dB	-38 dB	-44 dB

That's a pretty good drop off starting at 2 kHz and a look at the HRO-60 i-f bandwidth chart, attachment #2, shows the reason for this. If one hopes to hear hi-fi AM with an HRO-60 the tone control will have to be set to a more advantageous position! Even with this adjustment it is doubtful that one could get that close to a flat recovered audio response.

This test can be easily included in future measurements...if it seems worthwhile.

As an aside...looking at the recovered audio bandwidth and distortion measurements so far...one has to question the wisdom of relying on transmitted audio quality reports without knowing the receiver (and how it's adjusted) at the listeners end...       


   

[W1VD]

These may align better...

100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz	3 kHz	4 kHz	5 kHz	6 kHz
+1	+1	0	0	0	0	+1	+2	+1	+1	0

Results of the recovered AM audio bandwidth for the widest i-f filter are as follows:

100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz	3 kHz	4 kHz	5 kHz	6 kHz
+2	+2	+2	+1	0	0	-5	-16	-32	-38	-44

[Jeff W9GY]

Ah, yes ... LF audio distortion is related to the AGC system.  I remember this from my car radio design days...trying to compromise AGC time constant with acceptable LF audio distortion.  Get the AGC time constant too fast and the system tries to adjust the gain at an audio frequency rate, thereby creating the distortion.