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Circuit description: Fig. 1 shows the details of the up-convertor
circuit using a teflon-fibreglass microstrip construction and an
NE695 FET.
The LO (6 GHz) and IF (70 MHz) signals are applied to the
FET gate while the UC signal is taken from the drain output.
The LO matching in the input and the UC signal matching in
the output is accomplished by means of inductive and capacitive microstrip stubs. The IF signal is coupled through a 001 /iF
capacitor with IF choke providing the necessary isolation from
the gate DC bias supply. The IF is coupled at a zero impedance
point (LO frequency), so it does not detune the LO matching
circuit. A 6 pF DC blocking capacitor stops the IF from entering the LO port.
In the output, any IF signal is again stopped by the DC
blocking capacitor, while the LO and the unwanted sidebands
are filtered out by an external waveguide bandpass filter.
8 FOXALL, T. c , IBRAHIM, A. A., and HUPE, c J.: 'Minimum phase CCD
transversal filters', IEEE J. Solid-State Circuits, 1977, SC-12, pp.
9 SEQUIN, C. H., TOMPSETT, M. F., SUCIU, P. I., RYAN, P. M., a n d Z1MANY,
E. j . , JUN.: 'Self contained charge coupled split electrode filter using
a novel sensing technique', ibid., 1977, SC-12, pp. 626-632
10 IBRAHIM, A. A., HUPE, G. J., and FOXALL, T. G.: 'Double split electrode
transversal filter for telecommunication applications', ibid., 1979,
SC-14, pp. 80-84
0013-5194/81 /130435-03S1.50/0
115 dBm
IF input
Indexing terms: Microwave circuits and systems, Field-effect
9-5 dBm
Performance of an MIC FET up-convertor shows 3-5 dB conversion gain from 70 MHz IF to 6 GHz output, with 0 dB
conversion loss from the LO to the up-converted output. A 15
dBm output signal was measured from an FET capable of 21
dBm output in the amplifier circuit.
~ 11
3 10
Introduction: Although the FET has been used in the microwave down-conversion (mixer) circuits,1'2 with conversion
gain and low noisefigure,its use as an up-convertor has not yet
been reported. The purpose of this letter is to present the performance results of a medium-power 6 GHz up-convertor suitable for use in radio communications systems.
Theory of operation: The FET exhibits two nonlinear regions
in its operations: (i) the gate voltage near the pinch-off point in
the drain-current/gate-voltage characteristic; (ii) the drain voltage near the 'knee' of the drain-current/drain-voltage characteristic. The former corresponds to a large variation of the FET
transconductance, gm, while the latter depends on the drain
resistance modulation by the driving LO signal between high
and low values, corresponding to the saturated and unsaturated channel current flow.
The conversion gain for the two cases is approximately the
same and is given by
where Rd is the saturation value of the drain resistance and Cg,
Rg are the gate capacitance and resistance.
The conversion gain is lower by a factor of n~2, or 10 dB,
than the corresponding amplifier gain. The drain mixing in the
down-convertor has the advantage of a slightly lower noise
figure and the inherent LO-input signal isolation.
For up-conversion, the gate mixing will have the isolation
advantage as only a negligible amount of the up-converted
signal (UC) leaks back from the drain output into the LO
input port (gate). In addition, the gate mixing operates with a
lower LO drive level for a given up-converted signal (UC)
output level.
11 12 13 V, 15 16
LO input,dBm
Performance results: Fig. 2 shows the UC output level as a
function of the LO input power for two different IF inputs (9-5
and 115 dBm).
The UC output begins to saturate at 16 dBm LO input. This
is the input level at which the FET would also saturate when
used in an amplifier circuit. The conversion gain (IF to UC) is
greater at a lower IF input (9-5 dBm). It varies from 07 to 35
dB as the LO drive is increased from 12 to 16 dBm. A larger
UC output is obtained by increasing the IF input to 11-5 dBm.
A 14-3 dBm UC output is then obtained with 15 dBm LO
The higher IF input level resulted in lower LO to UC conversion loss: 0 dB at 12 dBm LO drive and 0-7 dB at 15 dBm
drive. LO to UC conversion gain was measured when the IF
input was increased still further. However, any further increase
in the IF input level leads to an unacceptable signal distortion.
For IF levels below 11-5 dBm the AM/PM conversion was less
than l°/dB. The DC power consumption was 350 mW.
Conclusions: The experimental results establish the suitability
of FET circuits for the high-level IF to microwave upconversion. Both IF to UC and LO to UC conversion gains
have been measured. The use of FET up-convertors offers an
attractive cost advantage in an all-FET transmitter for
microwave radio communications. The present FET upconvertor circuit could replace a diode up-convertor and the
first two stages in the FET high-power amplifier.
6 GHz
Fig. 2 Up-convertor performance curves
15th May 1981
Northern Telecom Canada Ltd.
9300 Trans Canada Highway, St. Laurent, Quebec, Canada H3C 3S1
LO input L
Fig. 1 Details of up-convertor circuit
ELECTRONICS LETTERS 25th June 1981 Vol.17
BURA, p., and DIKSHIT, R.: 'FET mixers for communications satellite
transponders'. 1976 IEEE-MTT international microwave symposium digest, pp. 90-92
2 BURA, p., and DIKSHIT, R.: 'FET mixer with the drain LO injection',
Electron. Lett., 1976, 12, (20), pp. 536-537
No. 13
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