Ramsey Electronics QRP-80 Manual de instrucciones descargar pdf (Pagina 21)

HR-80• 21

value of C4. This will cause the varactor diode to have greater effect

on the tuning. Try values of 33pF up to 100pF.

FREQUENCY STABILITY:

A kilohertz of drift is not a big deal in FM circuits or a shortwave

broadcast configuration, but ANY frequency instability is annoying

when trying to receive CW or SSB. The varactor - controlled Local

Oscillator is indeed a VFO, and therefore needs all the consideration

given to any VFO circuit. Even though this tunable oscillator design

takes good advantage of the NE602’s capabilities and offers nice

tuning range from a simple varactor circuit, we do not represent it as

suitable for transmitter frequency control or for demanding receiver

applications. In any ham VFO design, ANY unshielded or

uncompensated oscillator components are susceptible to the

influences of temperature change and of nearby moving objects. The

use of a suitable enclosure and secure mounting of the PC board

within that enclosure will maximize the stability of the oscillator. One

to two KHz of slow drift may be expected as components warm to

operating temperature; after that the oscillator is reasonably stable

OTHER METHODS OF DIAL CALIBRATION:

A easy way of making your own dial is to paste a small rectangle of

paper behind the tuning dial knob. Then as you tune in different

frequencies, pencil in on the paper the received frequency. Or a

simple logging scale may be affixed behind the tuning knob on the

front panel. A logging scale is simply a set of numbers for adjusting a

control. For example, 4 might represent 3.5MHz and 5.5 being

3.72MHz, and so forth.

DIRECT FREQUENCY READOUT:

A novel advantage of the simple direct conversion receiver is that a

general purpose frequency counter such as the Ramsey CT90 may

be used to give a direct readout of the oscillator frequency. (And we

don't mind suggesting that the CT90 makes an attractive, matching

companion to your Ramsey Kit mini-station in addition to its many

other practical uses!) While you could experiment with an RF pickup

loop or tuned circuit and counter preamplifier to boost the oscillator’s

tiny output to a level suitable for the counter, the most reliable

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Ramsey Electronics QRP-80 Manual de instrucciones Pagina 21