The AWA Journal - RADIO RAMBLINGS

RADIO RAMBLINGS

by Walter Lindenbach
c/o CCL Investments Ltd.
Box 75020, Westhills RPO
Calgary, AB Canada T3H 3M1
Please include SASE for reply.
Email: lindenbachw@shaw.ca

Modulation Products Where They Didn't Belong!

Fig. 1. A.M. signal modulated at low percentage by a 1 kHz tone. Top trace: modulating waveform; bottom trace: modulated carrier.

This is an experience I had while working for a small radio station in the 1960s. One of the three turntable channels in our main control room didn't sound right. There was a "wissh- wissh- wissh" sound -- something like an AM radio station when it is not tuned correctly. This sound produced no deflection on the VU meter, but it certainly could be heard.

Noises like that produce effects in our ears similar to a tone in the 2-4 kHz range, which includes the frequencies to which our ears are most sensitive. Being the Station Engineer, it was up to me to restore good music reproduction from that turntable -- and fast!

Clearly, there was something wrong with that turntable, yes? No! The matter turned out to be much more mysterious than that.

Fig. 2. Signal of Figure 1 as seen on a spectrum analyzer.

After exhausting all the possible causes that I could imagine relating to that turntable, I thought about our recently-installed second UHF STL (ultrahigh frequency studio-transmitter link). The first one operated at 450.7 MHz and the new one was at 455.7. And Glory Be --when either of them was turned off, the "wissh- wissh" from the turntables stopped

But however could that be? The UHF transmitter carriers were 5 MHz apart; how could they produce audio products? They were both transmitting program audio by FM -- could that have something to do with it? Oh yes!

The power output of the STL transmitters was 14 W, modulation was wide-band FM, and the antennas were Yagis with a gain of about 13 dB, giving an ERP (Effective Radiated Power) of 300 W. Over what bandwidth does wide-band FM produce modulation products?

Fig. 3. Same as Figure 1, but modulation approaching 100%.

An instrument called a spectrum analyzer provides an easy way to look at this situation. Like an oscilloscope, the spectrum analyzer portrays a signal waveform with amplitude on the vertical or Y axis. But while the oscilloscope displays time on the horizontal or X axis, the horizontal axis of the spectrum analyzer shows a frequency segment, typically a "center frequency" (identified as "CF") in the middle of the screen, with lower frequencies to the left, and higher frequencies to the right.

Let's look at some modulation displays on the oscilloscope and spectrum analyzer. Figure 1 shows an oscilloscope display of a signal amplitude-modulated at a low percentage with a single 1 kHz tone. The upper trace shows the modulating waveform; the lower trace shows the resulting amplitude-modulated radio-frequency carrier. Figure 2 shows the same signal on a spectrum analyzer. In Figures 3 and 4, the modulation now approaches 100%.

Fig. 4. Signal of Figure 3 as seen on a spectrum analyzer.

On the spectrum analyzer displays, the sidebands are the short vertical lines to the left and right of the carrier, which is the taller vertical line at "CF". They are displaced from the carrier by 1 kHz, which is the modulation frequency.

Notice that the sideband amplitude is greater at the high-modulation percentage than the amplitude at low-modulation percentage, and that the carrier amplitude does not change with modulation percentage. The shortest vertical lines at the extreme left and extreme right of the display are displaced by 2 kHz above and 2 kHz below the carrier. They are modulation products produced by distortion.

Now let's see the same spectrum analyzer displays, but with frequency modulation. Here (Figure 5) we see more than two modulation products, i.e. sidebands, at a low modulation level (now called "modulation index"). There are many more modulation products (Figure 6) at a higher modulation index. These are not distortion products. The oscilloscope displays are not shown because there are no amplitude variations to see.

Fig. 5. Spectrum analyzer trace of a frequency modulated a signal at low modulation level.

Theoretically, FM modulation products -- sidebands -- will occur over a very wide band, especially with a high modulation index and high modulation frequencies. However, for practical purposes, a "channel bandwidth," i.e. the range of frequencies permitted for one FM radio station, has been established at 200 kHz. If sidebands are limited to a narrower bandwidth, distortion rises to unacceptable levels.

Two FM UHF transmitters operating 5 MHz apart, modulated with normal broadcast program content, can produce sidebands at least 2.5 MHz away from the carrier. Therefore, sidebands will appear with frequency differences within the audible range. But they are still radio frequency signals. How can they turn into audio signals? The slightest amplitude nonlinearity in the turntable pre-amplifier will cause mixing, and the result is an audio signal.

Fig. 6. Spectrum analyzer trace of a frequency modulated signal at higher modulation level.

A broadcast turntable similar to the one that was interfered with is shown as Figure 7. The tone arm included a magnetic cartridge whose zero-level was about -60 dBm. The output impedance was about 250 ohms, so the zero VU level output voltage was about 500 uV. The"wissh- wissh" sound was more than 20 dB lower, so it was being produced by an interference of about 50 uV.

Almost certainly, a small LC low-pass filter at the turntable pre-amplifier input would have remedied the problem. But at that time I didn’t know much about filters for such frequencies, so I took another, harder way.

The cable from the magnetic cartridge pickup to the pre-amplifier was a single conductor with a single shield. This type of shielded cable is intended primarily for high-impedance phono pickups, that is for electrostatic, not electromagnetic, shielding. The output from a magnetic phono pickup with a 250-ohm output impedance is more vulnerable to electromagnetic interference.

The cable was about 2 feet long from the cartridge to the pre-amplifier. The wavelength of a 450 MHz radio signal is about 2 feet -- just right to make an effective antenna for the UHF STL transmitter signal.

Fig. 7. A broadcast turntable similar to the one that was interfered with.

What if the pickup pre-amplifier cable shield was not protecting the magnetic pickup signal from the UHF signal? This would allow radio-frequency voltages at both 450.7 MHz and at 455.7 MHz to enter the first grid (yes, we had tubes then) in the pre-amplifier? If the voltages were large enough to mix nonlinearly in the pre-amplifier, they could cause the "wissh- wissh" interference.

If the interfering radio energy could not be filtered out, it had to be kept out of the pickup cartridge pre-amplifier input circuit by other means. A piece of heavy braided-copper-shield cable was cut to the length of the single-shield preamp cable The twisted wire pair inside the shield was removed and the pre-amplifier cable placed inside. Now the pre-amplifier cable was double-shielded. Mercifully, the tone arm had a counterweight to balance the heavier cable.

When the magnetic pickup was reconnected to the pre-amplifier input, both shields were connected together and grounded at the pre-amplifier end only. The"wissh- wissh" sound was gone and I got paid that month!

A Comment on the April Column

A letter from Blake Hawkins of Land O' Lakes, Florida was a pleasant surprise and a bonus.

Blake, a retired A.M. and TV broadcast engineer, mentioned that there was a block drawing in a Gates catalog describing the "Diplomat" console. It showed the isolating resistors that I had added to eliminate the cross-talk to the program channel from the cue circuit. They had been part of the original design!

He believes that this is but one more example, of which he had seen many, of an "improvement" made by someone who did not fully understand the circuits he was working with.

Thanks lots, Blake; it is very satisfying to know this.

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