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Single-Sideband Filters [Ham-Radio 1968-08 8P]

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Single-Sideband Filters [Ham-Radio 1968-08 8P] single-sideband filters In a single-sideband transmitter, the signal I coming from the balanced modulator is not 0 m yet an ssb signal; it still has both sidebands. Although the carrier has been suppressed, it is a0 A called a double-sideband suppressed-carrier The filters required ; signal. The job of removing the unwanted -w Z sideband is left to a device known simply for generating a ssb signal g as a filter. In a communications receiver, incoming r; signals must be sorted out by the tuned cir- fall into .- cuits of the rf and i-f sections. These coil- -0 P. capacitor combinations may allow adjacent two general categories, signals through almost as well as the desired V10 ones; their response is too broad. Removing crystal and mechanical; g those unwanted "side" frequencies is the job of a filter. 2 4 In a single-sideband receiver, every con- here is a. version the signal goes through generates an 3 unnecessary extra sideband because of the m a description of both nature of the heterodyning process. To re- 7- cover the modulation, the ssb detector needs only the original sideband. The job of elimi- nating the unwanted sideband is turned over to-you guessed it-a filter. On the schematic diagram of a modern ham receiver or transmitter, the filter is 40 august 1968 identified merely by a box labeled F1, F2 or 60-dB point. The ratio between the two FLI, FL2, etc. The filter circuit is almost never bandwidths is called the shape factor. The shown. Nor is the type of filter indicated. If filter in fig. 1A has a shape factor of 5. A low you look inside the enclosure, you learn shape factor means the skirts are steep, mak- very little more. The filter is a sealed "black ing the filter respond strongly to the de- box" that has been plugged in or wired into sired frequencies-within the passband- the circuit. What's in it remains a mystery. and deeply reject signals on either side. A More important, of course, is what it does. shape factor of 1 is, of course, ideal; the From that standpoint, you can think of a skirts are vertical. In practice, a shape factor filter as a three- or four-terminal device of 2 or 3 is acceptable. which certain signals are fed into, and out of Consider the selectivity required for an which they come in some altered form. It is a-m signal. Since both sidebands are needed only a "black box" for all practical purposes. for proper demodulation, the receiver must First, then, let's examine what filters do in pass a bandwidth of at least 3.5 or 4 kHz- ham receivers and transmitters; then we can enough for intelligibility. That width passes explore what's in them. sideband products for voice signals up to 2 kHz. The filter represented by the response shaping the curves graph in fig. 1B is for a-m reception. It has a An important characteristic of any com- 6-dB response width of 3.75 kHz and a 60-dB munications receiver is its selectivity. That response of 10 kHz. The shape factor is about is its ability to reject signals on either side of 2.5-giving fairly steep skirts. a desired one, while passing it freely. The Both curves in fig. 1A and 1B represent selectivity you need depends on the kind of responses centered in the i-f passband. For signal-that is, the modulation it carries. example, if the i-f is 3.395 MHz, the filter in A CW signal, for example, has no modu- fig. 1A responds well to frequencies from lation. It therefore has no sidebands on each 3.3948 to 3.3952 MHz. The 400-Hz response fig. 1. Response curves for typical filters used ment. A for cw recep- tion; B for a-m and C for ssb reception. 2.1- kHz bandswitch is standard. I-F If I-F\ok" Q 0 0 side of the carrier. It consists of the carrier is spread 200 Hz to each side of the i-f cen- alone. A receiver with very, very narrow ter. The i-f section with the filter of fig. 1B selectivity can pick up the CW carrier (keyed, (3.75 kHz wide) responds well from 3.393125 probably, for code transmission) and avoid to 3.396875 MHz. The filter rejects frequen- interference from other, nearby carriers. One cies above and below; nearby signals can- of the best ways to attain such selectivity is not get through. with a special filter in the i-f amplifier. The For single-sideband reception, the selec- curve in fig. 1A graphs the response of an i-f tivity of the receiver can be narrower than section using a very narrow CW-only filter. for a-m, since only one of the sidebands is Filter characteristics are rated by their re- present. A bandwidth of 2.5 kHz is plenty. sponse at two points: 6 dB and 60 dB below The curve in fig. 1C shows the response of maximum. The response in fig. 1A is 400 Hz the filter in one commercial ssb receiver. Its wide at the 6-dB point and 2 kHz wide at the 6-dB response is 2.1 kHz; 60-dB response is august 1968 41 5 kHz. The shape factor is about 2.4. In a superheterodyne ssb receiver, every There's something else special about the frequency conversion creates two sidebands ssb filter in fig. 1C. Its response is not cen- from the single-sideband signal. That's be- tered on the i-f. The "center" of the filter's cause the local oscillator signal beats with bandwidth is off to one side or the other of the incoming sideband and produces both the i-f, placing any i-f signal down on either sum and difference frequencies. Following skirt, below the 6-dB point. Which skirt is the i-f amplifier stage, only one sideband is chosen depends on which sideband must fall needed for demodulation. The frequency of within the bandpass. If the upper sideband the filter (fig. 2A) is offset from the i-f as al- must be amplified, the i-f is placed on the ready described, to eliminate the extra side- lower-frequency skirt of the filter response. band that has joined the desired one. fig. 2. Ssb filters in re- ceiver (A) and trans- ??-, FIRST BINOP*SS mlflER MIxm - CIRCUITS mitter (0). I HET OSCILLATOR The rejection characteristic blocks the lower In the ssb transmitter, the chief job of the sideband. filter (fig. 28) again is to eliminate the un- wanted sideband. If well designed, it also filters at work removes any vestige of the carrier that might For ssb, a filter like the one in fig. 1C can be left by the balanced modulator. Succeed- be used in a transmitter or receiver. In ing stages of frequency translation re-create modern transceivers, a single filter is used a double sideband, but the two are far for both. Let's see how and why. enough apart that it is easy to get rid of the unwanted one with ordinary tuned circuits. fig. 3. If the dsb "carrier" (which In a transmitter, there is also a need to is suppressed) is placed in either switch from one sideband to the other. With position shown, one sideband is eliminated. a single filter, this is done by shifting the frequency of the carrier oscillator. Then, the 6d8-f-7-2.111Hz WOE signal that reaches the filter is on the other skirt. The sketch in fig. 3 gives you some ~deahow this works. If an upper sideband is desired, the 3.3964-MHz USB carrier-oscil- lator crystal (fig. 26) is activated. Even though the carrier is eliminated by the time the sig- nals reach the filter, the upper and lower sidebands fall on each side of the position shown (US8 "carrier") on the upper skirt of the filter response curve. The sideband fre- 42 august 1968 quencies higher than the "carrier" are atten- ers who develop the gear. A few of these uated drastically; those below are amplified. are diagramed in fig. 4. They are merely ex- (Don't be alarmed that this is called the amples of different ways filters are used in upper sideband; when the signal passes ssb receivers. through a stage of conversion after the filter, The circuit from the Drake R-4B has a the heterodyning process will "flip the side- variable-selectivity filter, with its center at band over," making it an upper sideband in 5645 kHz. The filter can be switched to four the transmitter output.) different bandwidths: 400 Hz, 1.2 kHz, 2.4 For a lower sideband, the other crystal is kHz, and 4.8 kHz. The first is for CW; the activated, generating the 3.3936-MHz carrier. last is for a-rn; the others are for various ssb - SELECTIVITY -.- - - - - - - - - .. ,' ,' f .;h 33 - rn 0 SWITCH - OFF AM DRAKE R-40 - 1.2 - WCIYE 3 - SSB 4.5 - CW \ '\ HAMMARLUND HO-145 TO NEXT FILTER - -CF AMPLIFIER fig. 4. Selectivity filters in a few cammercial receivers. The sidebands fall on both sides of the posi- reception conditions. Not shown is an addl- tion indicated as LSB "carrier." The sideband tional phase control that affects ssb recep- on the upper side is amplified, and that on tlon. the lower is attenuated. Again, the ensuing Another way to obtain variable selectivity frequency conversion flips the signal over with a single filter is shown iv the diagram and produces a lower sideband at the out- from the Hammarlund HQ-145 receiver.
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