1280MHz Video System

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This document contains information about the BestOfferBuy 1280 MHz "700mW" wireless transmitter/receiver, and my modifications to this system.

Video transmitter and camera.

Summary

The BestOfferBuy 1280 MHz video system includes a 700mW (advertised) video transmitter, video receiver, two power adapters, two 1280 MHz antennas, and connecting cables. The transmitter weighs 59.9g with case, and 14.8g after de-casing and adding a wire harness/smaller SMA connector.

Antennas

Skew Planar Wheel

Cloverleaf

Crosshair

The crosshair is a ~10dBi circularly polarized antenna consisting of two L-shaped pieces of wire on the center conductor and ground, 1/4 wavelength above a ground plane. Build instructions for this antenna can be found here. The dimensions used in the build are found in this post.

Filters

1280 MHz Cavity Filter

This filter was designed as a microwave filter project during EE-455 at Cal Poly. The filter is a single rectangular cavity filter, tuned for critical coupling and minimum insertion loss at the target frequency (1280 MHz), for use as a post-antenna pre-LNA filter to reduce the powers of cellular and land radio band interferers. A plot of the filter's insertion loss is shown below. The filter has a minimum 19dB rejection at the <1 GHz cellular bands, and a minimum 31.8dB rejection at the >1.7 GHz cellular bands.

Insertion loss and match of the 1280 MHz cavity filter.

Photos

Transmitter Testing

The output power and current draw was measured over the range of 10.2-12.6V at a fixed frequency of 1280 MHz (Channel F). Below is a table of output power, bandwidth, and current draw (of both camera + transmitter) vs. input voltage. The bandwidths at these voltages were measured using an Agilent N9000A Signal Analyzer according to the FCC's definition of bandwidth as stated in CFR Title 47, Part 97.3.[1]

Voltage (V) Current (A) Power (dBm) 26dB Bandwidth (MHz)
10.2 0.29 22.5 2.34
10.8 0.30 23.8 2.41
11.4 0.32 25.1 2.33
12.0 0.34 26.1 2.37
12.6 0.36 26.8 2.24

The power at each output frequency was measured (with an input voltage of 12.6V) and is listed below.

Channel Frequency (MHz) Power (dBm)
1 993 27.7
2 1023 28.6
3 1053 28.3
4 1061 28.2
5 1083 27.9
6 1101 27.7
7 1120 27.0
8 1141 26.8
9 1161 26.8
A 1180 26.7
B 1200 26.6
C 1221 26.3
D 1241 26.1
E 1257 26.4
F 1280 26.8

It would be recommended to put a low pass filter on the transmitter output to reduce harmonics to below -40dBc, as the 2nd harmonic (2560 MHz) is close to the 2.45 GHz band used by RC equipment, and may desense 2.4 GHz receivers. A basic coax notch filter was built to reduce harmonics at these two frequencies.

Notch Filter

A 2.45/2.56 GHz notch filter was designed in Qucs to reduce spurious power from the transmitter and prevent desensing of the RC receiver. A photo of the constructed notch filter is shown in the image gallery. The schematic for the notch filter is shown below.

Notch filter schematic for S-parameter simulation in Qucs.

The filter was constructed from RG-316 coax and tuned manually on the Anritsu MS4622B VNA. The resulting performance is shown below, compared to the simulated response.

Resulting simulated and measured data (s21,s11 simulated; s43,s44 measured).

The results show that the real filter has greater loss at the pass frequency, and lower rejection, although these are variables of the filter's tuning, which was selected to have two notches, one for the 2.45 GHz band, and another at the 2nd harmonic of 2.56GHz. The filter shows a minimum 42.6dB rejection over the 2.45 GHz band.

Modifications

Receiver

The receiver is identical in design to the 900 MHz receiver and will have the 17 MHz SAW filter installed to improve the noise figure of the system.

Transmitter

Photos

References

  1. Federal Communications Commission, "Title 47, Part 97, Sec. 3 Definitions," Oct. 2011. Available: http://www.law.cornell.edu/cfr/text/47/97.3