# UHF Phase Modulation

I would like to design a system that produces -10 degrees and +10 degrees phase shifts at 409.6MHz (Basically BPSK).

The way I'm thinking about approaching this is by using a DDS to generate the BPSK at 12.8MHz and up-converting using a mixer to 409.6MHz, filtering, and amplifying to +10dBm.

Is this the best way to do this? I have also read that I can use a 32x PLL multiplier... would that be a better way?

Factors I am taking into consideration:

1. Complexity to design circuit/pcb

2. Cost of components

3. Low phase noise, low spurs, emi, etc.

4. Using a 12.8MHz ovenized oscillator (as 12.8 x 32 = 409.6)

• What data rate are you wanting to run at? Commented Mar 27, 2013 at 17:38
• 500 bits per second Commented Mar 27, 2013 at 20:53
• According to Wikipedia, BPSK has two points at +90 and -90 (or equivalently +0 and +180) degrees. I am curious -- why would anyone want to use "-10 degrees and +10 degrees"? (Should this be a separate question?) Commented Jun 26, 2013 at 4:45
• @davidcary To reduce the signal bandwidth. Commented Jun 26, 2013 at 12:02
• My understanding is that transmitting at -10 and +10 degrees is mathematically identical to transmitting an unchanging pilot carrier at 0 degrees and simultaneously transmitting a normal BPSK signal at -90 and +90 degrees, where the amplitude of the BPSK signal is smaller than -- tan(10 degrees) of -- the amplitude of the pilot. Phil Karn seems to say that doesn't reduce the real signal bandwidth -- the frequency spacing required to avoid interference -- although it may appear to have a narrower bandwidth on a spectrum analyzer. en.wikipedia.org/wiki/VMSK Commented Jun 27, 2013 at 14:15

The best way to go about doing this is basically bypassing the DDS altogether and generating the baseband from a DSP and generating the symbols using a IQ Modulator. A VCO based PLL can be used to generate an LO for the modulator.

Additionally, the magnitude of the I & Q baseband voltages has the following relation:

Vq = -tan(10°) * Vi
Vq = -tan(-10°) * Vi


A design worth considering is a colpitts oscillator with an ADF4111 PLL controlling centre frequency via a varactor diode (other PLLs I'm sure would work just fine). Here's a simple picture of one and I'd replace the transistor with a BFR92A transistor. R4 should be shorted out for decent ops at 400MHz. It's an FM modulator and works nicely. The FM can turn to PM by differentiating the data.

The varactor would be in series with about 22pF from the top of the inductor. The other end of the varactor would be ground and the dc control thru a 10k from the PLL. This pretty much describes a circuit I once designed. I added an extra transistor fed from the base - this was an emitter follower output to a filter and simple antenna. It only produced 0dBm from a 3.3V supply so you may need to run it from a bigger supply or add an amplifier.

You can feed the data (after differentiating and amplitude reduction) via a small capacitor to the varactor. The downside of this is that it needs a little care seting up and if you are not hyper-bothered about the exact phase shift then it should be ok.

A further note about the ADF4111 - it needs a small MCU to set up its registers and a xtal for a frequency reference.

• this is an amazing circuit. How can I design other circuits like this? Is there a specific book you can recommend? Commented Mar 28, 2013 at 1:18
• @hassan789 No specific book to recommend I'm afraid to say - but maybe someone else can recommend one. Maybe you should start another question on this basis. I'm sure there'll be some good answers Commented Mar 28, 2013 at 10:22