900MHz Video System

From rev0wiki
Jump to: navigation, search

This document contains information about the HobbyKing 900MHz 100mW PAL video system, and my modifications to this system.

Video transmitter and camera.

Summary

The HobbyKing 900MHz video system includes a 100mW video transmitter, video receiver, two whip antennas, and a Sony 420 line PAL 1/3" CCD Camera, specifications listed as follows:

  • CCD sensor type: 1/3 color SONY CCD
  • Pixel: 500(H) x 582(V) (PAL)
  • Scanning system: Interlaced scanning
  • Synchronization: System: Internal synchronization
  • Horizontal resolution: 420TV line
  • Minimum Illumination 0.01LUX/F1.2
  • DSP+CCD: CXD3142R+405AK
  • S/N Ratio: 48dB
  • Gamma Modification: 0.45
  • White balance: Auto
  • Auto backlight compensation: Auto
  • Lens: 3.6MM
  • Audio: No
  • Input voltage: 9~12.6V (Cut out at 9.06V, measured)
  • Electric current 80mA (~100mA measured)
  • Electronic Shutter: 1/50 (60) ~ 1/100,000s
  • Video output: 1.0VP-P composite video
  • Operation Temp.: -20~50
  • Size: 38 x 38mm

I have replaced the larger but higher performance Sony camera for a smaller CMOS camera sold by BuyInCoins for $7.13 (incl. shipping). I replaced the old wiring harness with a shielded USB cable to reduce potential noise/interference. The voltage regulator contained in the cable was transferred to the new harness and heatshrunk. The harness going to the video transmitter was also braided to reduce potential noise/interference.

Transmitter Testing

The output power and current draw was measured over the range of 6-12.6V at a fixed frequency of 910MHz (Channel 0, two pins closest to SMA connector shorted/all pins open). The transmitter outputs between 92.7-207.5mW over the voltage range of a 3S LiPo (9-12.6V), with output power falling nearly linearly as voltage decreases. The video TX works down to 5.2V, although the output power drops to 16.6mW. The BuyInCoins CMOS camera operates well down to 6.2V, with degradation of signal down to 5.2V, where video becomes unusable. The camera draws 120mA (+/-10%) over the voltage range of 6-12.6V. Below are graphs of the output power and current draw/efficiency vs. input voltage.

Pout and Current for the video transmitter vs. input voltage.
Efficiency and Pout for the video transmitter vs. input voltage.

The bandwidths at various 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) Power (dBm) 26dB Bandwidth (MHz)
6.0 13.2 13.15
6.6 15.1 13.13
7.2 16.0 14.06
7.8 17.6 13.94
8.4 18.7 13.69
9.0 19.2 14.29
9.6 20.1 15.20
10.2 20.7 14.47
10.8 21.1 14.94
11.2 21.3 14.60
11.8 22.1 15.26
12.4 22.9 14.70
12.6 22.8 14.42

The harmonics were measured to a frequency of 7280MHz as shown in the following image. It would be recommended to put a low pass filter on the transmitter output to reduce harmonics to below -40dBc, as the powers of the 2nd (1820MHz, 13.2dBm) and 3rd (2730MHz, -4.88dBm) harmonics are close to the frequencies of GPS (1575.42MHz) and the 2.45GHz band used by RC equipment, and may desense these receivers. A basic coax notch filter was built to reduce harmonics at these two frequencies. The resulting harmonic powers with the filter in line are also shown below.

900MHz Video Transmitter harmonic powers (to 8th harmonic).
900MHz Video Transmitter harmonic powers with notch filter in line.

Notch Filter

A 1.575/2.45GHz notch filter was designed in Qucs to reduce spurious power from the transmitter and prevent desensing of the GPS and RC receivers. 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 exhibits higher loss at the pass frequency (though the simulation used the default coax loss, RG-316 loss values could have been entered for a more accurate simulation), lower attenuation at the GPS frequency, but a significantly wider rejection around 2.45GHz.

Receiver Testing

The receiver operates from 8-12.6V, drawing a current of 235-380mA. With a 17MHz SAW filter the sensitivity of the receiver is -XX.X dBm.

Reverse Engineering

The video transmitter is based around a Fujitsu MB15E07SL 2.5GHz PLL IC, which uses an 8MHz crystal reference to generate the transmit frequencies of 900MHz, 910MHz, 980MHz, 1.04GHz, and 1.1GHz. The PLL is initialized by an Elan EM78P153S microcontroller via SPI, using 19-bit frames. The first command sets up the PLL and reference (R) counter. The second command sets up the N and A counters, yielding an output frequency as follows:

$ f_{vco}=(M*N + A)f_{osc}/R $

The MB15E07SL uses a prescaler of 32/33 or 64/65, as selected in the first register. For 910MHz, the config registers are as follows:

Programmable Reference Counter Latch 0b0010000001100100001
Programmable Counter Latch 0b0101100011001111000

The settings are configured as follows:

CP Current +/-1.5mA
LD/fout Select LD Output
Phase Comp. Polarity Negative (FC = 1)
Prescaler 64/65
R Counter 400
N Counter 710
A Counter 60

The audio signal modulates a 5.5MHz oscillator, which in turn, is added to the overall FM modulation via varactor diode. The video signal is attenuated through a potentiometer and along with the PLL loop filter output, also modulates the varactor diode. The varactor makes up part of a VCO, whose signal is amplified through an intermediate stage and fed back to the PLL. This intermediate signal is then amplified once more to the final output power of 20dBm. The schematic for the entire video transmitter is shown below.

900MHz Video Transmitter Schematic.

To modify the video transmitter for 1280MHz, the bulk capacitor on the VCO was changed from ~8pF to 4pF, and the N Counter was changed from 710 to 999. The new Counter Latch is modified as shown below:

Programmable Counter Latch 0b0111110011101111000

Modifications

Receiver

The receiver has been modified to use a 17MHz SAW filter on the IF, decreasing the overall system bandwidth and thus lowering the noise floor. The RSSI output of the IF receiver has also been brought out to an RP-SMA connector on the outside of the receiver.

Possible future modifications include using a microcontroller to control the PLL to tune to arbitrary frequencies, and using the RSSI output along with a microcontroller to map out signal strength in the band, in order to find a clean channel to operate on.

Transmitter

The Elan microcontroller on the transmitter will be replaced by an Atmel ATtiny or MSP430 microcontroller, which can perform the function of tuning the transmitter to other frequencies, possibly on command of a servo channel, as controlled by the receiver/microcontroller.

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