wa2ise
Super Member
Getting better fidelity from AM radio stations.
The AM NRSC-1 standard is:
Contrary to popular belief, AM stations in the United States are not required to roll off audio above 5 kHz. For many years, no audio filtering was required, at all! In the early 1990s, however, a set of frequency response specs called NRSC-1 was adopted. Some type of standard was needed because, for years, many AM stations had been boosting the treble to compensate for the poor treble response of many tuners. This increase in treble had the side effect of causing more adjacent channel interference, which led radio manufacturers to further narrow the bandwidths of their tuners. The NRSC-1 spec specified a standard treble boost (pre-emphasis) curve. In addition to the treble boost, NRSC-1 required a sharp roloff above 10KHz. A related spec, NRSC-2 defined the amount of permissible emissions on nearby channels. The NRSC-2 requirements can be found in CFR 47, part 73.44. According to NRSC-2, any emissions 10.2KHz to 20KHz from the carrier frequency must be at least 25 dB below the carrier. Emissions 20KHz to 30KHz from the carrier must be at least 35 dB below the carrier, and so on.
If you have the typical modern digitally tuned AM/FM stereo receiver for your home audio system, you probably noticed the poor quality of the audio from the AM section of the tuner. No audio high frequencies at all (above about 4KHz). As stated above, AM stations broadcast audio up to 10KHz. Which makes their AM modulated signal have 20KHz bandwidth. The FCC assigns carrier frequencies further apart than this in your particular town. Out of town signals on adjacent channels are usually too weak to be heard on your local station. Most modern receivers use a ceramic filter of about 10KHz at most, yielding audio that tops out at 5KHz.
this mod applied to a Technics tuner using an AN7273 AM/FM chip. One issue I've run into is that I can make the filter look decent on the bench, using the sweep and marker generators and scope described below. But the response can get messed up installing it into the radio. So the use of buffer amp transistors, one on the input side and another on the output. This sort of thing is commonly done in defense electronics, where the cost is of little concern, but isn't done in consumer electronics. The output buffer also includes some gain to make up the insertion loss of the filter. The AM mixer output runs around 0.18ma and I can use 39K and 150K resistors to provide a reasonable load for it. Note that this buffer transistor uses a voltage supply (14V) that is higher than that feeding the chip (approx 8V). This should provide enough head room for AM radio signals coming out of the mixer. As the filter has a peak at the AM radio station carrier about 2dB higher than the edges of the bandpass (this is a feature in the sense that this peak will help avoid detector distortion due to insufficient carrier) I added a small cap to bridge an audio level dropping resistor used in AM mode, but shorted by Q212 when the tuner is in FM mode. This will make up for the droop in the filter, to bring up the treble on AM radio stations. This may make for poor phase linearity or group delay in the audio output, but the ear may not much care.
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A setup to look at passbands of filters
I'd like to see the passbands of various ceramic and other filters, and it turns out I have the equipment to make it happen. A frequency generator (VC2002 function generator), a sweep generator (Heath 1274 sweep function generator, and a scope (Tek TAS465). The setup:
I can adjust the amplitudes on both generators. I can see the shape of the bandpass (linear, not dBs though) and I can also see the amplitude of the marker frequency outside the passband of the filter on the scope. The scope is triggered from the sweep gen's "pulse" output (one pulse per sweep). The sweep is a frequency chirp, here from about 410 to 470KHz. The marker is always on. But the marker has to go thru the filter to get on the scope, and changing the marker frequency I can see when it drops to half amplitude and thus the 3dB points, high freq and low. The marker also beats with the sweep generator (which doesn't really show in the pictures) when the frequencies of the two happen to coincide.
The Device Under Test (DUT) is a ceramic filter at 455KHz, and has a little more than 20KHz bandwidth at 3dB. Which would be good for 10KHz of AM radio station audio, the max the FCC allows. Hifi AM.
Okay... Yes, channel spacing is 10KHz, but not in the same town. Thus you can have decent sound on your local AM music stations.
The AM NRSC-1 standard is:
Contrary to popular belief, AM stations in the United States are not required to roll off audio above 5 kHz. For many years, no audio filtering was required, at all! In the early 1990s, however, a set of frequency response specs called NRSC-1 was adopted. Some type of standard was needed because, for years, many AM stations had been boosting the treble to compensate for the poor treble response of many tuners. This increase in treble had the side effect of causing more adjacent channel interference, which led radio manufacturers to further narrow the bandwidths of their tuners. The NRSC-1 spec specified a standard treble boost (pre-emphasis) curve. In addition to the treble boost, NRSC-1 required a sharp roloff above 10KHz. A related spec, NRSC-2 defined the amount of permissible emissions on nearby channels. The NRSC-2 requirements can be found in CFR 47, part 73.44. According to NRSC-2, any emissions 10.2KHz to 20KHz from the carrier frequency must be at least 25 dB below the carrier. Emissions 20KHz to 30KHz from the carrier must be at least 35 dB below the carrier, and so on.
If you have the typical modern digitally tuned AM/FM stereo receiver for your home audio system, you probably noticed the poor quality of the audio from the AM section of the tuner. No audio high frequencies at all (above about 4KHz). As stated above, AM stations broadcast audio up to 10KHz. Which makes their AM modulated signal have 20KHz bandwidth. The FCC assigns carrier frequencies further apart than this in your particular town. Out of town signals on adjacent channels are usually too weak to be heard on your local station. Most modern receivers use a ceramic filter of about 10KHz at most, yielding audio that tops out at 5KHz.
this mod applied to a Technics tuner using an AN7273 AM/FM chip. One issue I've run into is that I can make the filter look decent on the bench, using the sweep and marker generators and scope described below. But the response can get messed up installing it into the radio. So the use of buffer amp transistors, one on the input side and another on the output. This sort of thing is commonly done in defense electronics, where the cost is of little concern, but isn't done in consumer electronics. The output buffer also includes some gain to make up the insertion loss of the filter. The AM mixer output runs around 0.18ma and I can use 39K and 150K resistors to provide a reasonable load for it. Note that this buffer transistor uses a voltage supply (14V) that is higher than that feeding the chip (approx 8V). This should provide enough head room for AM radio signals coming out of the mixer. As the filter has a peak at the AM radio station carrier about 2dB higher than the edges of the bandpass (this is a feature in the sense that this peak will help avoid detector distortion due to insufficient carrier) I added a small cap to bridge an audio level dropping resistor used in AM mode, but shorted by Q212 when the tuner is in FM mode. This will make up for the droop in the filter, to bring up the treble on AM radio stations. This may make for poor phase linearity or group delay in the audio output, but the ear may not much care.
-------------------------
A setup to look at passbands of filters
I'd like to see the passbands of various ceramic and other filters, and it turns out I have the equipment to make it happen. A frequency generator (VC2002 function generator), a sweep generator (Heath 1274 sweep function generator, and a scope (Tek TAS465). The setup:
I can adjust the amplitudes on both generators. I can see the shape of the bandpass (linear, not dBs though) and I can also see the amplitude of the marker frequency outside the passband of the filter on the scope. The scope is triggered from the sweep gen's "pulse" output (one pulse per sweep). The sweep is a frequency chirp, here from about 410 to 470KHz. The marker is always on. But the marker has to go thru the filter to get on the scope, and changing the marker frequency I can see when it drops to half amplitude and thus the 3dB points, high freq and low. The marker also beats with the sweep generator (which doesn't really show in the pictures) when the frequencies of the two happen to coincide.
The Device Under Test (DUT) is a ceramic filter at 455KHz, and has a little more than 20KHz bandwidth at 3dB. Which would be good for 10KHz of AM radio station audio, the max the FCC allows. Hifi AM.
