Step Recovery Diode Multiplier
for Modularized Spectrum Analyzer

     This page will act as a design guide for building a Step Recovery Diode Multiplier for the Modularized Spectrum Analyzer.  The MSA's 2nd Local Oscillator, which runs at 1024 MHz, is a combination of PLL 2 module and VCO 2 module.  Those two modules can be replaced with this SRD multiplier scheme, with improved Spectrum Analyzer phase noise results.

 This Page was Started Dec. 18, 2004
Updated 1-25-05, add link for CAD layout of SRD board

Go back to the MSA Main Page

SRDM Block Diagram to replace PLL2 and VCO 2:
SRD Multiplier Block Diagram
Here is a good pdf download from Agilent (HP) for using PIN diodes in place of SRD's http://cp.literature.agilent.com/litweb/pdf/5966-4998E.pdf

SRD Module :
SRD Module
Step Recovery Diode Module
    This module will respond to a sine input or the square CMOS clock from the 64 MHz Master Clock Module.  The first section of the 74AC04 will buffer the input.  The second stage will square up the signal even more.  The third stage is the driver to the SRD.  The parallel arrangement will assure maximum drive for the SRD.  The 64 MHz power from the parallel driver is approximately +16 dBm.  The 50 ohm R2 is the self bias resistor for the SRD, and it's value can be changed to optimize different SRD's.  Notice, there is no load or resonator on the output of D1.  The hairpin input of the cavity filter will act as a DC bias return for the SRD.  The hairpin is actually a transformer and acts as the load transducer for the SRD output.
    Now, the observant may notice that D1, MPN 3401, is not a step recovery diode.  It is a PIN diode.  Although not as efficient as an SRD, this particular PIN diode performs well in this design.  It measures only 3 dB less efficient than an HP 5082-0113.
    The actual measured output of the SRD module into a 50 ohm resistive load is approximately -23 dBm at 1024 MHz.  When the SRD passes through the pass filter into a load, the resulting load power is -5 dBm at 1024 MHz.  I tested 10 MPN 3401's in this circuit and the power spred was -3 dBm to -6 dBm, without changing the bias resistor, R2.
    I realize that this sounds like voodoo reverse-conservation of energy, but the cavity filter acts as a reflector to all harmonics except the one at 1024.  All those harmonics are directed back to the SRD and their power will increase the efficiency of the SRD action.
    In reality, power could be incresed somewhat by building a coaxial delay line between the SRD output and the input to the cavity filter.  I chose not to do this for the reason that it would not increase the level to +6 dBm or greater.  Theoretically, the maximum efficiency for a multiplier is  1/N, where N=16 (16th harmonic).  In this case 1/16 is 6.25%.  With a +16 dBm input, and 6.26% efficiency, the theoretical output would be +2 dBm.  Since a post amplifier is needed in either case, the delay line is deemed unnecessary.
    Here are screen shots of the SRD Module Board Layout.  I used ExpressPCB.  The size of the board is 1.4 x 1.35 inches.  Layer 1 is the silkscreen, layer 2 is the top copper, layer 3 is the bottom copper (as seen from top) :
SRD PIC LAYERS ALL SRD LAYERS 2 AND 3 SRD LAYER 2
Click for :  all Layers,              Layers 2 and 3,                          Layer 2,                    
    The board is layed out so that either the DIP or SOIC package 74AC04 can be used.

1024 MHz Cavity Filter for Multiplier, dimensions are inches :

1024 MHz Cavity Filter, 2 MHz BW

    This filter is used as the resonator for the SRD output at 1024 MHz, the 16 th harmonic of the SRD's 64 MHz input.  I won't repeat the build process on this page.  For pictures and construction techniques, go to the Cavity Filter page.

Post Amplifier for Multiplier
amplifier module
SRD Post Amplifier Module
    This is the same amplifier module shown on the Modularized SA page as the Final I.F. amplifier.  It can be used here by not populating the input low pass filter.  Delete the two 20 pf capacitors and 100 nH inductor.  Just bridge over the space that would take the inductor.  Use a 100 nH choke for the amplifier bias.  This amplifier module has about 19 dB of gain at 1024 MHz.  The output will be saturated with an output of about +13 dBm, when the input is -7 dBm or greater.

    Here are a few notes on how the Step Recovery Diode (or PIN Diode) works:
Using the SRD module as a test bed, with only a 50 ohm resistive load on its output, the left photo is what the output waveform looks like (with a 3.33 MHz square wave input signal).
Pic of SRD output  Pic of SRD output 2
    Disregard the small positive going glitches.  These are line reflections with my test set-up.  The right photo is the same output, but with a second trace when the SRD diode is bypassed with a short. When the negative portion of the square wave is applied to the SRD, it acts like a normal diode.  In the center of the right photo (5 cm), you can see that there is about .7 volts difference between the negative square wave and the SRD output.  This is the normal forward bias voltage drop of a diode.  As the square wave input is transitioning from positive to negative, there is a noticeable negative overshoot on the SRD output (4 cm and 10 cm).  This is attributed to the high junction capacitance of the SRD when in forward bias.
    When the square wave input transitions from negative to positive, the SRD will conduct for a short period of time, even though the SRD is biased in the reversed direction.  This is called the Carrier Lifetime.  The lifetime for this particular device is about 10 nsec.  The positve overshoot is, again, attributed to the junction capacitance of the SRD (1 cm and 7 cm).
    The carrier lifetime is created by minority carriers conducting in the SRD in the normal forward direction.  When the diode is immediately reversed biased (the square wave transitions from neg to pos), the minority carriers will continue to conduct current, even though they are conducting in the reverse direction.  When they have lost their energy to conduct (at the end of the Lifetime), the diode will quickly cease conducting (1.2 and 7.2 cm). This is not exactly an immediate action.  The time it takes to cease conducting is called the Transition Time.  The faster this action occurs, the steeper the falling edge of the signal.  This action of the transition is what will create the many harmonics of the input signal.  A very fast transition time will create many more higher frequency harmonics.  I cannot measure this particular SRD's Transition Time, but it is likely on the order of 100 psec or less.
    If the module's 50 ohm load resistor is replaced with an inductive load, the pictures above, that are showing the SRD voltage output, can be thought of as a current output into the inductive load.  Basic electrical theory says that if you create current flow through a coil, and then immediately remove the current, the magnetic lines of flux collapse and cause a voltage spike across the inductor.  This is exactly what we are doing by installing the cavity filter.  The hairpin input of the cavity is an inductor.  Even through it is a short, straight piece of wire, it has high Inductive Reactance at 1024 Mhz.
    This final picture is the same SRD device, but, instead of driving it with a square wave source, it is driven with a 13 MHz sine wave.  The action is exactly the same as the square source.
Pic of SRD sine input