A receiver having a 1 mhz per band tuning rate would require 30 bands to tune the entire SW spectrum. With a crystal controlled front end this would require 30 crystals. Reduce the tuning rate to 500khz (as common for a ham band rig) and you would now need 60 crystals. For a military receiver such as the R390, the cost was no problem, But for a civilian rig, something had to give! Amateur receivers, covering only 5 bands would only need five crystals (with the extra crystals for full ten meter coverage a modest option) so hardly a second thought was given here.
When National brought out the last of their HRO series receivers in the '60's (the HRO-500 and the very rare HRO-600) they took a different route. The first hfo was replaced by a PLL (phase locked loop) frequency synthesizer. This device uses a variable frequency oscillator which is locked to a crystal reference oscillator. This vfo is modified by having a voltage variable capacitor (usually a solid state diode, but tubes have actually been used here. Such a tube circuit is known as a reactance modulator) in the tuned circuit. A sample of the vfo output is compared with a selected harmonic of a crystal oscillator in a 'phase detector' circuit and the resulting error signal is applied to the voltage variable capacitor. This 'feed back loop' is the servo mechanism that locks the vfo (now known as a voltage controlled oscillator) on frequency. In the HRO-500 the vco was tuned by a front panel dial to the nearest reference harmonic where it would 'lock on'. The user changed bands by tuning the vco to the next reference harmonic. Such an oscillator has appeared in the ARRL handbook in the past.
Enhancing the dial readout to provide more accuracy was another task. As the cost of TTL digital logic fell manufactures started adding frequency counters to receivers. The vfo frequency was read directly, and the display adjusted by the if frequencies to provide direct readout of the air frequency to the nearest hz in some cases.
Another way to tune a vco/pll combination is to select the desired harmonic by use of a modulo-n divider. If the output of the vco is first feed into a divide-by-n circuit and the output of the divider is feed to the phase detector then the vco can be tuned by changing the divider ratio. This gives us a digitally tuned oscillator. This is the circuit that is used today to select frequencies in amateur FM transceivers, CB transceivers, and even most of today's AM, FM radios and TV sets.
The digitally tuned PLL is limited in the size of its' frequency step, or tuning speed. The time required for lock up is proportional to the step size. The smaller the step size, the longer the lock up time. This is not a big problem for a channelized radio, but it prevents the PLL from being used in a continuously tuned radio. For SSB the smallest practical step size is 100 hz, as any larger tuning error would render the signal unreadable. A PLL with a step size of 100hz would have too long a lockup time to allow us to 'scan the band' as we do with a vfo controlled rig. Even so, there was such a rig made. The last transceiver built by the descendant of Swan used a PLL-VFO with a tuning rate of 100 hz /step. This rig was NOT very popular (and was Swan's swan song). Yet the PLL CAN be used in a rig to provide vfo like tuning by using two or three of them. If two pll's are provided, one of which tunes in 10khz steps, and a second which tunes in 10.010khz steps, we can use the difference between them to provide a 10hz tuning rate. This is exactly what Icom did in their early 700 series radios. It does take a small micro computer to control the two oscillators and provide a digital display (plus allows us to have several memories for quick frequency recall) so these rigs have one or more microprocessors on board.
Yet another use of the digital pll was in combination with a true vfo. In these circuits the receiver is tuned by a vfo which is used as part of a variable reference along with a digital divider and a vco. A frequency counter provides a dial display. Such a combination synthesizer provides the hfo function for a rig using a hf first IF. The Kenwood TS830 was an example of such a radio.
Even though modern communications receivers now have their functions
controlled by microprocessors it has been necessary to retain the
familiar analog controls that receivers have always had. Tuning a radio
is done with a large knob on the front panel. The Swan pll tuned radio
previously described had a spring loaded pot with a center detent for its'
tuning knob. As the knob was rotated clockwise the frequency display
moved upscale. The more the knob was moved the faster the radio moved
up in frequency. The radio tuned down in frequency when the knob
was turned counterclockwise. Since this is not the kind of
control most of us are used to, other manufactures have sort a way to retain
a convention method of tuning receivers. The knob turns a metal disk
with many holes or slots along its circumference. These holes interrupt
the path of light beams between two light emitting diodes and two
photo transistors, producing two trains of pulses. The spacing of
the led-photo transistor pairs is such that the two pulse trains are 90
degrees apart, or in quadature. A simple circuit (or microprocessor
software) can operate an up-down counter such that a clockwise rotation
of the knob runs the counter up, while a counterclockwise rotation runs
the counter down. The output of the counter is presented to the microprocessor
to adjust the frequency of the synthesizer used as the vfo in the receiver.