DARTS Transponder

Engineering Page

Last Updated: September 10, 1998

Transmitter

The transponder's transmitter consists of a gated computer clock oscillator, a MMIC multipler, two printed-circuit bandpass filters, and two MMIC amplifiers. The transmitter can be disabled by the on-board computer, allowing the interrogator to command a specific transponder to transmit.

An 80 MHz computer clock oscillator is used to drive a step-recovery diode, generating copious odd-harmonic output. The harmonic at 5760 MHz (or thereabout) is quite strong. A 3-pole microstrip filter isolates only the harmonics in that region.

Following the filter is another MMIC amplifier, and a 5-pole microstrip filter. The final filter effectively isolates only the desired harmonic (others are more than 40 dB down). The filter output is both isolated and amplified by a final MMIC stage up to the 0 dBm level, which drives the final amplifier stage.
 

SRD Details

Back-to-back SRD's seem to increase harmonic output, as does driving the SRD's directly from the 80 MHz oscillator (no coupling capacitor).
 
 

Multiplication Factors
Source
 Input Freq
Output Freq
Factor
Factor (-dB)
Computer Clock Osc
 64 MHz
5760.0 MHz
90
19.54
Computer Clock Osc
 80 MHz
5760.0 MHz
72
18.57
SAW Oscillator
 418.0
5852.0 MHz
14
11.46
PLL Synth
 1440.0
5760.0
4
6.02
 

SRD multiplier components

Micrometrics has some interesting data in a PDF file, including the figure below.
 
 
 
 
Component
Function
High-Pass Filter (Input)
Restrict harmonic content to SRD ?
Bias Network
Establish negative bias to enhance snap pulse
Matching section
?
Impulse generator
Generate sharp forward-bias spike -- not needed if input is square wave?
Idler Circuit
?
Output Matching Section
?
Output filter
Restrict output harmonic content to desired band
 

More Power

I need more input power.  All the SRD stuff I've seen says the SRD likes to be driven with between +15 and +26 dBm into 50 Ohms.  No way my thing is providing that.
 

So--Why Doesn't It Work?!?

 I don't think my design has all these components--and that may be why it doesn't work.

Question--why did my first SRD multiplier -- an oscillator feeding an SRD, coupled by a capacitor -- work so well?  How did the "pipecap filter" transmitter work?

I have some idea about the pipecap transmitter.  I think the pipcap served as a nice resonator for the SRD.  Also, I think I used three stages of amplification, one of which was the MMIC-of-doom, the MGA-86576.

 

 Pulse Modulator

The DARTS pulse modulator circuit is used in both the transponder and the interrogator.

The circuit has three functions:

  1. Delay the incoming trigger pulse (RXPULSE) 12.8 uS before transmitting the output pulse.  This function is only significant in the transponder, where it is used to generate a fixed ehco delay.
  2. Generate a pulse whose width depends on the state of the TLMDATA line.  If the TLMDATA line is low, a pulse 12.8 uS wide is generated.  If the TLMDATA line is high, a pulse 6.4 uS wide is generated.
  3. Modulate the 80 MHz clock with the pulse in a phase-coherent fashion.
The output (MOD80MHZ) consists of a train of 80 MHz (12.5 ns period) pulses either 12.8 uS or 19.2 uS long.  This translates to a count of either 1024 or 1728  pulses at 80 MHz.

In addition, the circuit has a transmit disable input (TXDIS), which disables the MOD80MHZ output.
The BEACONPULS input allows the circuit to be triggered by the transponder's microprocessor instead of the receiver, to create regularly spaced beacon pulses which aid in finding the transponder.
 

Crystal-Video Receiver

RF Toolbox was used to calculate its tangential sensitivity.

(Note:  Set browser width so that the following ASCII figure has a box around it.)

              TSS CALCULATION with PRE-DETECTION GAIN
¦------------------------------------------------------------------------------¦
¦                                                                              ¦
¦                       ¦\                           ¦\                        ¦
¦     RF amplifier      ¦__\        Detector         ¦__\    Video Amplifier   ¦
¦                       ¦____\                       ¦____\                    ¦
¦        In  o----------¦_____>----------------------¦_____>--------o Out      ¦
¦                       ¦____/          ¦            ¦____/                    ¦
¦                       ¦__/          +---+          ¦__/                      ¦
¦                       ¦/            ¦/ \¦          ¦/                        ¦
¦                                     +---+                                    ¦
¦     Gain        32.0   dB             ¦                                      ¦
¦     N.F.         1.9   dB           -----     N.F.         4.0   dB          ¦
¦     Bandwidth   80.000 x 10^6 Hz     ---      Bandwidth   10.000 x 10^6 Hz   ¦
¦                                                                             ¦
¦------------------------------------------------------------------------------¦
¦   Enter either:                                ¦  Or:                        ¦
¦   Detector  mV/mW  =    0.0 and Ohms =    0.0  ¦  Quality factor =  245.97   ¦
¦------------------------------------------------------------------------------¦
¦                 <<<   Calculated TSS = -83.48802 dBm.   >>>                  ¦
+------------------------------------------------------------------------------+

RF amplifier is a composite of two stages:
 
MGA-86576 ERA-2 COMPOSITE
NF (@ 6 GHz) 1.8 6.5 1.95
Gain (@ 6 GHz) 18  14 32
 

The pre-detector bandwidth limitation is provided by the patch antenna itself.

The equation is:  (from Jasik's Antenna Handbook)

   BW = 4 f^2 (t / (1/32))
 
   f in GHz
   BW in MHz
   t is board thickness, in inches.

Detector is a voltage doubler based on HP HSMS-2852 dual zero-bias detector, which is
desirable w/r/t cost (0.91 each).  Prototype will probably use (2) Alpha DDC254-250's
are on hand but have differing characteristics.

Detector figure of merit:

                    M  =  B/(r)^.5

      Where B       is detector sensitivity in volts/watt
            r       is detector effective source resistance
 

M= 245.97 for the HP HSMS-2852  (notice the doubled value is used above, since it's a voltage doubler.)

  The calculation is based on the work of W. J. Lucas, as reported in
  Proceedings of the IEEE, 113, 1321-1330 (1966).

  The specific form of the equations used is that presented by James B.
  Tsui, in his book Microwave Receivers with EW Applications, Published
  by John Wiley and Sons.

The video amplifier is as of yet undetermined, but a 4 dB NF was guessed.


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This document copyright Steve Bragg, KA9MVA.