Page last updated: 21/03/2005

This page presents an add-on internal prescaler circuit for the HP 5328A frequency counter. The Prescaler connects as a front-end to Channel C (520MHz) to provide a frequency coverage from 100MHz to more than 12GHz. Actual frequency is displayed, not a divided ratio.

The HP 5328A Frequency Counter is a work horse. This is a well engineered 9-digit, full featured counter with various options such as a DVM, frequency extension to 520MHz or 1.3 GHz, ovenized TCXO and HP-IB  (GPIB). This counter is readily available on Ebay and in hamfest for less than $100.  These features make it attractive for various modifications.

Adding a microwave prescaler to this counter adds a lot of value to the instrument. What makes this project feasible is availability of the Hittite HMC363S8G. This is a 1GHz to 12GHz, divide-by-8 prescaler chip. The component's front end provides a sensitivity that compares with commercially available microwave counters. At the other end of the range, the chip can tolerate input power of +13dBm. Its price is around $8 USD, which is reasonable even for the hobbyist.

IMPLEMENTATION

The idea behind the prescaler implementation is the following: If you divide down the input signal frequency by a factor N, and you divide down the counter timebase (reference) frequency by the same ratio N, the counter will still read and display the right frequency. In other words, the counter will count fewer input pulses by a factor N, but the sampling window is expanded by the same amount N, yielding the same frequency measurement. The only drawback to this technique is that the sampling period required to obtain a specified resolution is lengthened by that same factor N. This does not really constitute a problem in practice though.

In the case of the HP 5328A, the following criteria were taken into account when selecting the division ratio N:

• The maximum frequency expected is 12-15GHz, this limit being set by current prescaler chip technology,
• The maximum frequency accepted by the existing C channel on the HP 5328A counter is 520MHz,
• The maximum frequency accepted by the A and B channels is 125MHz,
• The desire to use only one model / one brand of prescaler chip to simplify ordering,
• Availability of the prescaler chip via distributors,
  

A compromise value of N=64 was selected. This is obviously achieveable by using two instances of  the same divide-by-8 prescaler chip model.

The figure below shows how my prescaler is implemented in the HP 5328A counter.

The Prescaler project is made of two main circuits (PCBs). The figures below show the two schematic diagrams of these PCBs.

The RF Prescaler Schematic Diagram A MMIC amplifier can be added between C4 to C11 to increase sensitivity. If so, care must be taken to avoid overloading U1, the first prescaler chip.	Click on the figure to enlarge it.
The TTL Divider Schematic Diagram JP1, the 4x2 header, can be replaced with a simple wire jumper soldered in place.	Click on the figure to enlarge

The Prescaler board layout This PCB is made of double-sided copper clad glass-epoxy materal of 0.031" thickness. This thickness is important to guarantee a 50 ohm impedance on the high speed traces. The copper pattern resides on the PCB top side along with the components. A solid copper ground plane must be present on the bottom side. The ground plane should cover the whole surface except for the zone underneath the copper fingers at the bottom. See the picture below. Drilling of holes is required in marked locations and are only for ground via connections. This PCB installs in the 5328A's slot XA7.	Click on the figure to enlarge it.
The TTL Divider board layout This PCB is made of single-sided copper clad glass-epoxy materal of 0.062" thickness. This copper pattern resides on the PCB bottom side. The components are located on the top side. Drilling of holes is required to solder the components to the copper pattern. This PCB installs in the 5328A's slot XA5A.	Click on the figure to enlarge it
The Channel C Enable Signal Pickup board layout This PCB is made of single-sided copper clad glass-epoxy materal of 0.062" thickness. This PCB installs in the counter's XA5B slot. It picks up the "Channel C Enable" signal.	Click on the figure to enlarge it

Prescaler PCB Copper Layout Prescaler_PCB.pdf

Divider PCB Copper Layout Divider_PCB.pdf

Channel C Enable PCB Copper Layout ChanC_PCB.pdf

• All three PCB layouts have a copper outline to mark the edges and ease the cutting process. These copper outlines should be trimmed off, otherwise short circuits will occur.
  

• This board uses surface-mounted components. Only experienced individuals should assemble it. A very fine soldering iron tip and a magnifying glass are required.

• Assembly should follow the following order:
• SMA connectors,
• Ground vias, except for the 4 vias above and below each prescaler chip,
• All capacitors,
• The two prescaler chips (see note below),
• Remaining ground vias near the prescaler chips.
  

• When soldering the SMA connectors to the pcb edge, ensure you keep the air gap between the connector and the PCB edge to a minimum. Also, bridge the gap between the bottom ground plane and the connector ground pins using plenty of solder.
  
• The two prescaler chips are modified SOIC-8 package and have a ground pad on their bottom side. These must be soldered to the PCB ground island first. To do so, apply a thin amount of solder to the chip and a thin amount of solder to the copper island. Do not overheat the chip. Drop the chip in place. Heat up the PCB with the soldering iron as close as possible to the chip while maintaining the latter in position. Then solder the eight pins.
• Solid pieces of AWG 20 or 22 wire should be inserted in marked locations to act as ground vias. They should be soldered to both PCB sides and then cut flush with the surfaces.

• Solder a piece of wire between the Divider board's "Channel C Enable" pad and the longer finger on the Channel C Enable Signal Pickup PCB.
• The use of DIP IC sockets is optional but recommended.
  

The Prescaler PCB - Top View (First Prototype)	Click on the figure to enlarge it.
The Prescaler PCB - Bottom View (First Prototype)	Click on the figure to enlarge it.
The 10MHz Divider PCB and The Channel C Enable Signal Pickup PCB (First Prototypes)	Click on the figure to enlarge it.

Access the botton side of the HP 5328A counter motherboard PCB in the 10MHz reference area (rear-left area when seen from above the counter). Cut the trace that links U16-pin8 to U17-pin13.

Click on the figure to enlarge it.

Access the top side of the HP 5328A counter motherboard PCB, in the 10MHz reference area (rear-left area when seen from above the counter). If the counter is equipped with the HP-IB option, first unscrew the HP-IB board (including the rear HP-IB connector and lift the board. Disconnecting the board is not required. Solder a RG-174 coax cable to U16-pin8. Solder the other end of the coax cable to the TTL Divider board 10MHz input pad. Solder another RG-174 coax cable to U17-pin1. Solder the other end of the coax cable to the TTL Divider board 10MHz output pad. Only solder the coax shields at the Divider PCB end. Ensure that the shields are insulated at the counter's end. If necessary, re-install the HP-IB option board.

Click on the figure to enlarge it.

Insert the TTL Divider board in slot XA5A as shown in the picture. Components must face the right handside when looking from the front of the counter. Insert the Channel C Enable signal pickup PCB in slot XA5B as shown in the picture. The copper fingers on the PCB must face the right handside of the counter when looking from the front of the counter and finger 0 should be closest to the front of the counter.  The wire should come out near the top edge of the PCB.

Click on the figure to enlarge it.

Insert the Prescaler board in slot XA7 as shown in the picture. Components and copper traces must face the left handside when looking from the front of the counter. Since the PCB in only 0.031" thick (half the thickness of a typical PCB, you must also insert an insulated shim of equivalent thickness on the back of the PCB finger area. A piece of plastic or PCB material (copper removed) can be used. Connect the Prescaler RF output SMA to the Existing Channel C input cable using a SMA-SMB coax cable/adapter combination. Hook up the Prescaler Input SMA to a RF connector previously installed on the faceplate.

Click on the figure to enlarge it.

The HP 5328A Prescaler Operational Range

Click on the figure to enlarge it.