Local Oscillators for Samplers

Local Oscillators for Samplers

T.S. Ikin


A programmable Local Oscillator has been provided for each Sampler Module available to COBRA. These have a fixed output power at nominally +15dBm and a variable frequency in the range 20MHz through 83.75MHz, in steps of 250kHz. They are all phase locked to the station 5MHz standard. The output power is designed to meet the requirements of the downconversion circuits in the samplers.


(Background information) Communication is via RS-232 link between LO and Interface board. The RS-232 channel is handled by Microcontroller The frequency range is delivered by two VCO's which are switched in by a relay. Station clock 5MHz is delivered to each LO module via BLVDS on a backplane. Please note that toward the lower frequency range, the second harmonic becomes obtrusive. It is expected that the hardware filters on the sampler boards involved will take care of this, so that no out-of-band noise is recorded.


Programming of these LO's is simply by means of the 8 bit words available in the Interface board connected to the COBRA Adlink boards. Each interface board communicates to two sampler modules, and two LO modules. There is one 8 bit word per LO.

Frequency (MHz) = 20 + ( 0.250.n )
where n is the 8 bit number sent to the LO module.

Examples: For 20.00MHz, the code programmed is 0 For 83.75MHz, the code programmed is 255 d ($FF)

Click here for a complete code/frequency list

The address table relating to each Interface Board/Adlink Card is as follows:

Address $00Presets for Sampler0
Address $01Presets for Sampler1
Address $02Presets for Interface
Address $03Presets for Interface
Address $04Presets for LO0
Address $05Presets for LO1

The address must be sent to the upper 8 bits of the PB word, and the programmed data to the lower 8 bits of the PB word.

Where A(7:0) is the register address, and D(7:0) is the Data Byte to be stored in that register