7 GHz prescaler

Why we need a prescaler?
For most home-brewers, there is always a frequency counter on the bench, with which we can make some precision measurement during our project building. The commonly used counter has upper frequency limit of 220MHz, 600MHz or 1100MHz. But nowadays HAM frequency frontier has been extended to 249GHz. For a RF experimenter, the practical frequency is under 12GHz, including the popular 10.245GHz. So we need to extend the frequency range we can measure. The most practical way is to design a front end prescaler for our frequency counter.
How fast can we get?
So far as we can afford, the prescaler with highest input frequency of 25GHz is available. That is the 25673DV-LGA from the Inphi Corp. But it costs about $150... Do you still want to use it? OK, let's find some alternative parts. The HMC493LP3 from Hittite with InGaP GaAs HBT technology can work up to 18GHz, and the price is reasonable for some critical applications, but still too high for a homebrewer. OK, Let's keep looking for ... In fact, the most practical device is still based on Silicon, that includes: the MB501~MB511 from Fujitsu, the uPB1507 from NEC and MC12079 from Motorola. The highest available frequency is about 3GHz. But most of these ICs contain modulus of 64/128/256. That is not convenient if we use them as range extender of our on-hand frequency counter. ( Some genius HAM had designed a great divide-by-1000 circuit using a modulus 128 chip, please reference this site: http://www.qsl.net/n9zia/pre/index.html ) So still need to ... ...
Configureable and versatile?
As a radio amateur, we do not have to use the component as it's normal function, as in this design. The main role is the ADF4107 PLL chip from Analog Device. This IC has upper frequency limit of 7GHz, and fortunately it has a selectable output pin of the RF divider output, that's just what we need.

External RF signal with amplitude between -20dBm and +10dBm is fed into the analog front end of the IC. The amplified signal is fed into the divide by N circuit. This circuit consists of a programmable dual modulus divider and two counter registers. One register is the 13bit B register, the other is the 8 bit A register. The divider factor N is derived: N = B*P + A, where P is the modulus number, either 8,16,32 or 64. With different modulus number, the duty cycle of the output signal is different. Followed the divider is the MUX circuit. This versatile circuit can route different signals to the MUXOUT pin, and here we select the N divider output.

The ADF4107 is a complete PLL IC with complex function, but here we only care about several things:

(1) Make sure the chip is working under normal operation, not in idle mode.

(2) Initial the A register and B register to set the divide number to be 500, here I use the by 16 modulus, the N = 500 = 31 * 16 + 4, so Reg A = 4, Reg B = 31.

(3) Make sure the N divider signal is routed to the MUXOUT pin.

To do this, we configure the chip as following:

Reference Counter Latch: ..........HEX 00 00 04

AB Counter Latch:.......................HEX 00 1F 11

Function Latch:.............................HEX 40 00 22

With this configuration, the chip works as a 7GHz prescaler with division number of 1000, then we can read the frequency counter with unit of GHz.

Structure design:
As an add-on front end to a FC, the unit should be battery powered to simplify the wire connection and to shrink the case size. Here I use a 3.6V Ni-MH rechargeable battery and also there is a simple charging circuit on the PCB. All the initialization is fulfilled by an AT89C2051 MCU. On start up, the MCU write the ADF4107's registers with correct serial bus timing. Then the MCU itself goes into idle mode to save power. Within the measurement range, the current consumption is less than 20mA. The unit can work more than 6 hours with one full charge.
Project Files:
Schematic: PCB:
MCU code:
The BOX:
Measurement and Test:
Figure not available at current time
Lowest available frequency 80MHz
Figure not available at current time
Highest frequency 7.2GHz
Sensitivity at 1GHz & 6GHz

Output from ADF4107 MUXOUT

Output from 74HCT74

Readout of synthesized 1.073 740 GHz Signal