In planning any ‘bandspread’ improvement, it is useful to understand exactly how varactor tuning works. The control level varies the amount of DC voltage applied to the diode. The highest produces the lowest capacitance, while the lowest voltage yields the highest capacitance. The spread is typically about 30 to 40pF. from minimum to maximum.The goal in achieving bandspread or ‘finetuning’ is to achieve a smaller variation in capacitance with the comfortable tuning of a knob.

Any resistor in a range from 500 ohms to 3000 ohms, connected in series with the ground lead of R2, will set a limiting effect on varactor capacitance range and therefore create a “bandspread “ effect. The higher the resistance value, the shorter the available tuning range.

If you wish to reduce the tuning range of R2 down to 40 or 30KHz or even less for a favorite band segment or calling frequency, reducing the value of C4 will decrease the capacitance effect of the varactor diode, D1. Values from 2 to 5pF may give you exactly the tuning range you are looking for.

If you are using your own enclosure and have room for a separate fine tuning control, one useful choice is a 250 ohm speaker attenuation rheostat (Radio Shack model no. 271-265) wired in series with either outside ‘leg’ of the main tuning control. Such a control can make fine adjustments to a CW signal so the tone is pleasant and smooth sounding.

If you want the opposite of bandspread (that is as MUCH frequency range as possible over full rotation of the tuning knob ) increase the 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 SA602’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

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Ramsey Electronics HR40 manual Frequency Stability
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HR40 specifications

The Ramsey Electronics HR40 is a versatile and high-performance FM transmitter that has gained recognition among hobbyists and professionals looking to broadcast audio signals. With its robust features and user-friendly design, the HR40 stands as an excellent choice for a range of applications, from low-power broadcasting to personal audio projects.

One of the standout features of the HR40 is its frequency range, typically allowing for transmission on the FM band from 88 to 108 MHz. This broad range ensures compatibility with a variety of receivers and allows users to select an optimal frequency for their needs, minimizing potential interference. The transmitter is designed to offer excellent audio fidelity, with a frequency response that meets and exceeds industry standards, ensuring clear and crisp sound quality.

The HR40 utilizes advanced modulation techniques, which contribute to its performance in terms of signal clarity and range. The use of frequency modulation allows the HR40 to transmit audio signals with minimal distortion, even at higher volume levels. Additionally, the transmitter is equipped with a built-in audio preamplifier, which enhances audio quality by providing a clean and powerful signal before transmission.

Another notable characteristic of the HR40 is its adjustable output power. Users can modify the power level to suit their specific broadcasting needs, whether for indoor use or outdoor applications. This flexibility is particularly useful in managing transmission range, allowing users to tailor their setup without excessive overkill or underperformance.

The compact design of the HR40 also stands out, making it a popular choice for those who need a portable broadcasting solution. The unit is lightweight and easy to transport, while its durable construction ensures longevity and resilience in various environments.

In terms of ease of use, the Ramsey Electronics HR40 offers simple installation and operation. It comes with a clear instruction manual, allowing users to set up their transmitters quickly. The intuitive interface and minimal setup requirements enable beginners to jump right in, while advanced users can take advantage of its expandability and customization options.

Overall, the Ramsey Electronics HR40 combines user-friendliness with a range of powerful features aimed at both amateur and experienced broadcasters. Its excellent audio quality, adjustable output power, and compact design make it a favorite for many seeking reliable FM transmission capabilities.
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