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Radio
Broadcast Technical Consulting and Sales
10032 Island Drive, Brainerd, MN
56401
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Radio
Broadcast Technical Consulting and Sales |
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Technical Tips from Mark W. Persons |
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Dayton AF315 AM Receiver Modifications |
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This is a Dayton Industrial AF315 AM Receiver. It is a very good choice for EAS monitoring, off-air monitoring, and now for AM/FM repeaters. |
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One
problem I ran into was poor low-frequency audio response and high
distortion on audio frequencies below 100 Hz. The situation got
get worse as the received signal strength increased. After some
troubleshooting, I determined that the receiver AGC (Automatic Gain
Control) was the culprit. It was trying to change receiver gain
while following low frequency audio. The fix was to install a 1000
mfd/25 VDC capacitor in the AGC circuit to slow it down. Here, it is connected to
one of the "S" pins, which are sometimes used for an external signal
strength meter. The positive side of the capacitor goes to the "S"
pin.
An additional problem I noted was a peak in audio frequency response of about 6 dB at 180 Hz. The capacitor solved that problem too. |
| But wait, there is
more to the story! You can choose one of two IF (Intermediate
Frequency) filters when ordering a receiver, and one of two post-detection de-emphasis audio
filters by jumper option in the receiver. This chart gives measurement data on a typical
Dayton AF315 Receiver and compares it to the NRSC (National Radio Systems
Committee) recommendations that were later adopted by the FCC. The
plan is for audio to be pre-emphasized (high frequency boosted) when it
is broadcast and de-emphasized (high frequency attenuated) at the
receiving end to reduce noise when audio is heard. If the standard
is followed exactly, the result is flat frequency response. Most radios today have the audio high frequency response rolled off sharply after 3 or 4 KHz to reduce noise and listener complaints. They use narrow band IF components by design. Wide band IF receivers can only work well when the received signal strength is relatively high and when there is little nearby noise to interfere with the desired signal. Testing was done at the M. W. Persons facility in January of 2011 while using the above mentioned 1000 mfd capacitor in the AGC circuit. |
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Audio Frequency Response |
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Wide Band IF |
Narrow Band IF |
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Audio Frequency |
NRSC Standard |
50 us de-emphasis |
75 us de-emphasis |
50 us de-emphasis |
75
us de-emphasis |
| 30 Hz | 0 dB | -5.8 dB | -5.8 dB | -5.8 dB | -5.6 dB |
| 50 Hz | 0 dB | -2.5 dB | -2.6 dB | -2.6 dB | -2.5 dB |
| 100 Hz | 0 dB | 0.0 dB | 0.0 dB | 0.0 dB | 0.0 dB |
| 1 KHz | -0.75 dB | +0.9 dB | +0.4 dB | +1.3 dB | +1.2 dB |
| 2 KHz | -2.5 dB | -0.1 dB | -1.1 dB | 0.0 dB | -0.7 dB |
| 3 KHz | -4.3 dB | -0.1 dB | -3.0 dB | +2.3 dB | -3.8 dB |
| 4 KHz | -5.6 dB | -1.4 dB | -4.8 dB | -4.7 dB | -6.6 dB |
| 5 KHz | -6.9 dB | -4.1 dB | -6.4 dB | -17.2 dB | -19.3 dB |
| 6 KHz | -7.8 dB | -5.6 dB | -9.0 dB | -29.7 dB | -32.0 dB |
| 7 KHz | -8.6 dB | -7.1 dB | -9.6 dB | -40.7 dB | -41.8 dB |
| 8 KHz | -9.3 dB | -8.6 dB | -11.8 dB | -45.7 dB | -46.8 dB |
| 9 KHz | -9.7 dB | -11.2 dB | -14.2 dB | ||
| 10 KHz | -10.0 dB | -14.6 dB | -17.3 dB | ||
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The stories go on and on.
Stop in again sometime. I'll leave the soldering iron on for you.
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page last edited 01/09/2011
