PhaseLock SSB

Introduction
These days it's uncommon but in the eighties of the previous century when the number of HAMs was increasing strongly as well did electronic devices of often not that great of quality,
more and more Low Frequency Detection short LFD troubles where becoming a problem.
This was especially the case when VHF (2M) SingleSideBand radio's increasingly became popular among radio amateurs.
SSB even more than AM strongly varies in amplitude causing LFD on any semiconductor device that is not somehow protected against it.
As auntie Mary's new stereo was loaded with semiconductors and long speaker cables formed an excellent antenna system you'd be sure ghostly sounds where coming from the speakers even when the radio was switched off.

Frequency modulated signals do as the name says not vary in amplitude resulting in far less noticeable radio interference.
For this reason in the eighties when CB-Radio was legalized in Europe the approved mode was FM and limited output power.

Phase-Loop SSB
The Dutch radio amateur Klaas Spaargaren PA0KSB (at least i think it was him) came to the idea to build a SSB exciter that was followed by a phase locked oscillator.
This way the output was equivalent to the frequency part of the modulation but the output was constant.
It technically became a FM signal that could be received using a normal communications radio with its mode set to SSB.
A SSB compatible FM transmitter was the result.

A simplified representation of a PLL-SSB transmitter.

(Dis)Advantages
Although the concept really worked, this technology as well had it's disadvantages.
We're now talking about a FM signal of which it's bandwidth theoretical is unlimited.
The advantage of distributing the available power over a small signal had disappeared.
Still the advantage of a fairly good signal to noise ratio at the receiver side was kept intact.
Also due to the missing amplitude information the modulation always was at the same very loud level,
like the operator had switched on a speech processor.

On the other hand some interesting new applications where found.
At the time it was a real tough job to create SingleSideBand signals with reasonable power on the SHF bands like 10GHz.
For this there basically the following options:

(1) Use gunn diodes, as used for burglar alarms
This offered only very small power output and only FMmish or AMmish modulation.

(2) Use a klystron from old radar equipment
This offered some more output but also neither
meat nor fish regarding to the available AM or FM mode.

(3) Creating a lot of power on lower bands like 13cm and multiply it up to 10GHz using varacter-diodes.
This could offer a reasonable amount of power and descent AM, FM or CW modulation.

(4) A traditional transverter and a amplifier using traveling wave tubes.
This offered an all mode solution for the price of a expensive motorcar.

Using the PhaseLock trick it was possible to create a FM transmitter which was phase-locked to a SSB-Exciter,
effectively having a SSB signal on this otherwise 'unreachable' band.
I have never tried this myself, the few klystrons i had happily have been adopted by a hording collector
and i never used my 3C39 coaxial valves either.

Building and Using PhaseLocked-SSB equipment
Since technically we have a frequency modulated radio-transmitter,
the output is constant during the complete transmission.
This is different from normal SSB, and it is important to keep this in mind regarding the size of the heat-sink of the power-amplifier.
On the other hand, the PA does not need to be linear, a C-class module will do fine.

Some years ago a have setup a PLL-SSB transmitter using GNU-Radio and a USRP followed by a power-amplifier derived from an old mobile car-phone system, offering an output of about 20 Watts.
Apart from the PA which only worked on 70cm, the system worked with ease on 2M, 70cM, 23cM and 13cM.

PhaseLock-SSB using GNU-Radio and a USRP.

First response was 'Way over-modulated !'.
Well it was not, by concept PLL-SSB does not have amplitude information and therefore can't be over-modulated.
Indeed it behaves like a badly set speech processor, and during speech pause you can hear a mouse passing by.

Other experiments
A related pretty autistic project I've been working on was a standard 22 Channel CB radio on which i replaced the PLL chip with a FPGA, such trying to modulated the VCO in a Phase-Loop-SSB mather.
The goal was not to make changes to the PCB other than replacing the PLL-Chip (a PLL08).
Receiving would be done by feeding the 455kHz IF into a ADC creating a basic SDR receiver.
Although promising the CB-radio fell in decay dramatically by the many experiments i had to do
and such i lost interest of the project.

A article i wrote about this subject for a Dutch online magazine can be found on the following link.
DARU #20