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The HP 310A can operate as a VLF/LF/MF receiver. It can demodulate AM, USB and LSB using the Weaver method of quadrature demodulation. This also allows the 310A to demodulate CW, FSK (eg: RTTY or FAX) and MCW (used on LF navigation beacons).
The 310A is a superheterodyne receiver with an IF of 3MHz. This does lead to image problems at frequencies of less than 500kHz. A low pass filter at the input will eliminate these images. A suitable circuit s decribed below.
Operation is straightforward. The mode switch is uused to select AM, USB or LSB, the bandwidth is selcted from 3kHz, 1kHz or 200Hz and the coarse tuning knob is rotated until the desired frequency is reached. The fine tune knob allows fiinal tuning to take place with accuracy.
The Weaver quadrature demodulator produces a heterodyne whistle when tuning ino an AM station. Users may find this annoying, but it is best to tune the receiver to zero beat to eliminate the whistle and ensure accurate tuning. The narrow 3kHz IF bandwidth produces connsiderable distortion to the audio, especiially music, so the 310A could never be an adequate entertainment receiver.
It does become an excellent monitor or DX receiver. The reading on the mechanical "digital" frequency indicattor can be reliedon as the carrier frequency once zero beat is reached.
The Fine Tune control allows SSB to be tuned satisfactorily and the 3kHz IF bandwidth is adequate.
Use either USB or LSB and the 1kHz bandwidth for RTTY ACII or other telegraphy. MCW is bbest received with the AM setting.
Use either the USB or LSB moode and the 200Hz bandwidth. There will be a frequency offset depending on your tuning.
This should be as sensitive as possible, without overloading the receiver. Use the voltmeter as an indicator At these low frequencies noise is extremely high, often in the millivolt (or volt!) range.
The AGC performance is quite poor as would be consistent with the 310A's design as a frequency selectived voltmeter. This can make listening quite fatigueing, as will the high noise level at these frequencies.
In the frequency band 9kHz to 1500kHz propagation is almost exclusively by ground wave. Ground wave may be thought of as the sum of the surface wave and the space wave. Polarisation is exclusively vertical in this mode. Ground wave mode propagation shows no eviidence of fading.
Below 100kHz some transmissions may be propagated by the so-called "ionospheric wave-guide mode" whereby the surface of the Earth and the D layer of the atmosphere act as the boundaries of a waveguide. Polarisation is exclusively vertical in this mode.
Aboove 500kHz night tiime propagation may take place by space wave mode reflected from the D wave. Fading distinguishes this mode, as a result of mutual interference between the ordinary and the extraordinary rays.
In all three modes vertically polarised receiving antennas are the usual choice. If the experimenter lacks sufficient real estate to erect a Beverage antenna, the choice narrows to the loop antenna or the voltage probe. Loop antennas have been adequately described elsewhere. Voltage probes are less familiar.
This is simply a vertical conducting rod with the lower end attached to a cable. The rod should be mounted as high as possible. Mine is mounted 10m above ground in a station sited 60m above sea level. Reception is excellent.
Every AM broadcaster within the area east of Ceduna, west of Dunedin, north of Launceston and south of Thursday island can be received. Since almost all AM broadcasters in Australia have a clear channels allocated to them, with only a few low power regional stations having to share frequencies, reception is excellent.
A five pole Buutterworth provides a critical frequency of 1700kHz with only 3dB ripple in the pass band.
HP 310A Specifications Controls Maintenance Receiver Spectrum analysis Distortion Networks
Copyright John Alexander Faulkner VK2DVW 28 October 2004
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