Functional Requirements

The transmitter must frequency modulate a input digital signal into S-band and output at least 2.5 Watts of power into a patch antenna.

Overview

The transmitter designed for SRP-4 was built by Stephen Bruss. S-Band transmitters are used because the Poker Flat Research Range (PFRR) supports S-Band equipment.

Electronic interface Requirements

Carrier frequency Range: 2.2 ¡© 2.3 GHz
Frequency Stability: 0.001%, 10PPM
Voltage 24-32 V DC
Power output: 3 W
Power consumption: 18 W at 28 V
Current 50 mA (max) at 28 V
Modulation: FM
Deviation: +/- 8 MHz max
Deviation linearity: 2.5% max. BSL for +/-8 MHz deviation
Start up time: < 5 seconds
Antenna compatibility: 10:1 VSWR
Harmonic/spurious response: -40 dBc
Input impedance: 220 §Ù
Input voltage: 1 V peak-peak
Frequency response: 1 dB; 3 dB
Presentable under-voltage and over-voltage RF system shutdown

Design Issues

According to Stephen Bruss's suggestion, 1 V p-p is a good input voltage level. Since the transmitter uses AC coupling, as long as the input level is limited in 1 V p-p, the voltage range inside the transmitter will be ¡©0.5 V to +0.5 V. No voltage match circuitry is required.
Although the specification says the nominal impedance is 220 §Ù, it is significant only for a high frequency input signal. In SRP-4, the data rate is approximately 40 kHz.Even if the impedances do not match, the reflection will not influence the input. Therefore, impedance match circuitry is not necessary here.
Digital NRZ code is used. At the receiving end, the digital regenerator can recover the synchronous signal from NRZ code.
Because the modulating signal in SRP-4 is digital, the transmitter modulation linearity will not be a problem. The input digital signal bit rate will be no more than 300bps. For NRZ signals, the baseband bandwidth will be no more than 300kHz. In this range, the frequency response is flat regardless of the setup of the attenuators. The pulse width will be no less than 1/300 kHz = 3.3 us. According to Bruss, a 10ms long continuous DC signal will cause errors. This means when 3000 continuous 1s or 0s are transmitted, the DC response problem will be significant. However, 3000 continuous bits of the same value is so improbable that it can safely be ignored.
Data rate (hereafter, data rate means the total bit rate from the flight computer, including frame overhead and correction coding) is 40 kbps. The center frequency of the transmitter (and consequently the whole system) is to be set to 2.2155 GHz. The frequency range of the transmitter is designed to be 2.2-2.3 GHz (with the stability of +/-22 kHz). This frequency range was chosen because it is a designated frequency for telemetry at Poker Flat. PFRR has four designated FM telemetry bands with 16 MHz bandwidth, and eight 3 MHz wide bands. The center frequency of the patch antenna was set to 2.2155 GHz, with a bandwidth of 16 MHz. Although SRP-4 has a relatively low data bit rate and 16 MHz is probably not necessary, to match the patch antenna a center frequency of 2.2155 GHz is chosen.
In the Mission Initiation Conference, NASA pointed out that there might be a conflict in the 2.2155GHz channel. Further investigation is required and it may be necessary to change to the 2.2355GHz channel. Then the patch antenna would have to be re-designed.

The frequency deviation is set at 300 kHz. In FM modulation, if the modulating signal band has the maximum frequency of max f , and the maximum frequency deviation of the modulator is f , the bandwidth of the RF signal can be estimated by Carson's rule:

B=2*(deltaf+fmax)

Where B, deltaf, and fmax are all in Hz.
In order to reduce the noise at the receiver input, the minimum bandwidth is expected. On the other hand, wider bandwidth is also desired to achieve more FM improvement.