P-COM microwave oscillator modifications by Goran Popovic, AD6IW

There are lots of P-com oscillators available on the surplus market. Many people use PCOM PLL as parts, and some folks use the amplifier sections only, although most of them don't modify anything and use them as is. There is a need for a stable and good oscillator for 10GHz or 24GHz, as LO, beacon, signal source, test equipment, or something else.

fig 1 fig 2 Please click on the image to enlarge

After carefully inspecting the PLLs, and reverse engineering the PLL, I finally came up with an idea on how to modify the PCOM oscillator. In the process of selecting which of the oscillators to modify (because PCOM has many versions of PLL), I selected two of the most common types from the pile. (There is a frequency posted on the sticker.)

I selected a PLL between 9 and 11.7 GHz for modification. This type of PLL has a 5x multiplier. Some people modify the PCOM PLL's frequency just by reprogramming it via a microcontroller. In that case the output signal is locked to the right frequency but the PLL suffers from high phase noise and instability. Originally, the PLL was designed (and useful) for only wideband FM (data). This is the cause of the difficulty in trying to use this P-Com PLL in weak signal communications.

The original was built with a 10MHz TCXO which is not very useful for microwave.It NEEDS TO BE MODIFIED OR REPLACED TO HAVE BETTER PERFORMANCE (please see footnote).

The PLL loop filter is not optimized, and it has a noisy LM741 in it. The VCO is built with CRO and two BFG67 transistors. The oscillator output frequency is in the range of 2GHz, and tuned by a varactor diode. I measured the original VCO gain, and I decided to design a better loop filter.

First Modification, The Simple One

Step one:

Remove the original control board. Desolder the 9 connection points, fig 5. Remove the regulators, DB15 connector, LED and tantalum caps. These parts should be used on the new board. Fig 3, 4, 9.

fig 5	fig 3	fig 4	fig 9
Please click on the image to enlarge

Clean the whole unit with alcohol or acetone afterwards.

From the RF board, remove the LM741 including the filter parts, prescaler, and few passive components. Fig 6, 7, 8 The new PLL board is the same size and shape as the original control board.

fig 6 fig 7 fig 8 Please click on the image to enlarge

On the RF board was added an RF splitter, a low noise op amp, and a mmic as a buffer. Pic osc_mod.

ps pins Please click on the image to enlarge

The PLL chip is programmed to 1987.2MHz, which generates 9936MHz output frequency. This frequency is mixed with a 432MHz IF signal to get 10368MHz.

fig 10 fig 11 Please click on the image to enlarge

On the controller board there is a potentiometer for the RF output level adjustment. Any output level between 0 and 15dBm could be set and tapped from two SMA outputs.

All GaAs on the RF board are of type ATF13336. The SMA connector on the control board is for external 10MHz reference clock input. PIC 10

Second Modification

Remove the control board.

On the RF board, remove the original 2GHz oscillator with all the supporting components. I built a new PLL board which is the same size as the original contol board. The new PLL board utilizes a Stratum 3 reference oscillator, with low phase noise VCO, and very low phase noise PLL chip. This new design has the additional advantage of a very compact PLL. There are no separate grounds unlike the original design, and no inter-stage board connections.

fig 12 fig 13 fig 14 Please click on the image to enlarge

With a short piece of coax cable, the PLL output signal is injected into the RF board There are two MMICs,one is used as a buffer and the other as a level amplifier. The PLL, VCO, MMIC all operate at 3V. The PLL chip doesn't need a micro or a clock, because it runs configured from an EPROM. Red LED indicates a PLL lock , 10 turns of the potentiometer is for the RF output level adjustment from 0 to 15dBm. The output frequency on this oscillator is set to 10224MHz, or 10368MHz.

Results:

The described PLL oscillators are finally tested on 3cm ssb transverter. They work very well. The frequency stability is within +/- 1Hz. The phase noise very much depends on the reference oscillator. A good reference oscillator is recommended.

Presentation and demo was made for the 50MHz and UP group.